<PAGE>   1
                                                                  EXHIBIT 10.18

                                SECOND AMENDMENT
                         SERVICES AND SUPPORT AGREEMENT


                  This Second Amendment to Services and Support Agreement (the
"Second Amendment") effective as of October 1, 1998, is by and between Coast
Dental Services, Inc., a Delaware corporation ("CDS") and Coast Florida P.A., a
Florida professional services corporation (the "Dental Practice Entity").

                                R E C I T A L S

                  WHEREAS, CDS and Dental Practice Entity are parties to that
certain Services and Support Agreement dated as of October 1, 1996 and first
amended on June 1, 1997 (the "Services and Support Agreement"); and

                  WHEREAS, CDS and the Dental Practice Entity are now desirous
of modifying the Services and Support Agreement in accordance with the terms of
the Second Amendment.

                  NOW, THEREFORE, for good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged by the parties, the
parties agree to modify, supplement and amend the Services and Support
Agreement as follows:

                  1. Recitals. The statements contained in the 
Recitals of facts set forth above are true and correct and are by this 
reference made a part of the Second Amendment.

                  2. Service and Support Fee. Exhibit "A" to the Services and
Support Agreement is hereby amended to change the S&S Fee to Seventy-four
percent (74%).

                  3. Other Provisions. All of the terms and provisions
contained in the Services and Support Agreement shall remain in full force and
effect unless specifically modified, supplemented or amended by the Second
Amendment.

                  IN WITNESS WHEREOF, the parties to the First Amendment have
hereunto set their hands and seals on the day and year first above written.

                                    "DENTAL PRACTICE ENTITY"

                                    COAST FLORIDA P.A.



                                    By: /s/ Adam Diasti   
                                       ----------------------------------------
                                            Adam Diasti, President

                                    "CDS"

                                    COAST DENTAL SERVICES, INC.



                                    By: /s/ Terek Diasti 
                                       ----------------------------------------
                                            Terek Diasti, 
                                            Chief Executive Officer

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                                                                  EXHIBIT 10.18

                               THIRD AMENDMENT TO
                         SERVICES AND SUPPORT AGREEMENT

         This Third Amendment to Services and Support Agreement (the "Third
Amendment") effective as of February 1, 1999, is by and between Coast Dental
Services, Inc., a Delaware corporation ("CDS") and Coast Florida P.A., a
Florida professional services corporation (the "Dental Practice Entity").

                                R E C I T A L S

         WHEREAS, CDS and Dental Practice Entity are parties to that certain
Services and Support Agreement dated as of October 1, 1996, amended on June 1,
1997, and further amended on October 1, 1998 (the "Services and Support
Agreement"); and

         WHEREAS, CDS and the Dental Practice Entity are now desirous of
modifying the Services and Support Agreement in accordance with the terms of
the Third Amendment.

         NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged by the parties, the parties agree
to modify, supplement and amend the Services and Support Agreement as follows:

         1. Recitals. The statements contained in the Recitals of facts set
forth above are true and correct and are by this reference made a part of the
Third Amendment.

         2. Service and Support Fee. Exhibit "A" to the Services and Support
Agreement is hereby amended to change the S&S fee to Seventy-three percent
(73%).

         3. Deletion of Section 3.4. The Services and Support Agreement is
hereby modified to delete Section 3.4 of the First Amendment related to the
payments to the Dental Practice Entity for added internally developed Dental
Centers.

         4. Other Provisions. All of the terms and provisions contained in the
Services and Support Agreement shall remain in full force and effect unless
specifically modified, supplemented or amended by the Third Amendment.

         IN WITNESS WHEREOF, the parties have hereunto set their hands and
seals on the day and year first above written.

                                         "DENTAL PRACTICE ENTITY"

                                         COAST FLORIDA P.A.



                                         By: /s/ Adam Diasti
                                            ----------------------------------
                                                 Adam Diasti, President
                                         "CDS"

                                         COAST DENTAL SERVICES, INC.



                                         By: /s/ Terek Diastti
                                            ----------------------------------
                                                 Terek Diasti, 
                                                 Chief Executive Officer