Clinical, prospective, randomized double-blind trial will be conducted at the Federal Hospital Bonsucesso (HFB), Rio de Janeiro, Brazil. Eighty female patients, olther than 60 years, ASA I to III, undergoing laparoscopic cholecystectomy will be studied. They will be randomized into two groups of 40. One group will receive Palonosetron 75 mcg (Group P) and the other will receive Ondansetron 4mg (Group O). Patients will not receive premedication. Will be monitored in the operating room with non-invasive blood pressure, cardioscopy, pulse oximetry, capnography and bispectral index (BIS). After cannulation of peripheral vein, 10 ml of blood will be collected for further analysis. They will be given one minute before induction of anesthesia, intravenous (IV) bolus preparations of the studied antiemetics. Patients will be pre-oxygenated with oxygen at 100% for 5 minutes and the anesthetic induction is performed with intravenous administration of fentanyl 3 mcg / kg; Lidocaine 1.5 mg / kg and propofol 2 mg / kg. Tracheal intubation will be facilitated after 3 minutes of administration of Rocuronium 0.6 mg / kg EV .Maintenance of anesthesia will be with Sevoflurane 2 L / min in 50% oxygen / air, with its concentration adjusted to maintain BIS between 40-60. Remifentanil 0.05 mcg / kg / min to 0.2 mcg / kg / min IV can be administered intraoperatively if the heart rate or blood pressure to rise more than 20% of values baseline. Additional doses of rocuronium may also be administered as needed. Patients will receive paracoxibe 40 mg,dipirone 50 mg / kg and 50 mg of ranitidine IV after tracheal intubation and have the surgical wound infiltration with 20 mL of ropivacaine 0.5%, before the sutures. Neuromuscular blockade reversed with neostigmine 0.04 mg / kg and atropine 0.02 mg / kg IV at the end of surgery. Will be limited to insufflation of the pneumoperitoneum to abdominal pressure of 15 mmHg.

A non-participating physician will be aware of the research on which antiemetic was given and be responsible for postoperative prescription of patients. The researchers did not have access to prescription and medical records within 48 hours after surgery. Both groups will recive metoclopramide 10 mg (EV) as rescue medication for PONV, and the Group O will continue with prescription ondansetron 4 mg (EV) regular 8 / 8h, in the following 48 h after surgery. Patients will receive clinical visit by the research team 2, 6, 24 and 48 h after the end of surgery and will be questioned about the frequency and intensity of PONV, as well as other side effects. Finished the last evaluation, researchers will review the prescription and will note the use of opioids (type, dose, route of administration), use of rescue medication, total cost of antiemetic therapy (including 5HT3 antagonist and rescue medication), based on "Brasíndice" table.

All patients have their blood samples analyzed in the clinical research laboratory of the Universidade Federal Fluminense, for concurrent research of prevalence of polymorphisms of 5HT3 receptors and their correlation with the antiemetic effect. DNA will be extracted from blood samples using a kit to genomic DNA purification. Polymerase chain reaction test for real time will be applied below for analysis of single nucleotide polymorphisms of. The reactions are prepared with Genotyping according to the manufacturer's instructions and performed on a sequence detection system using standard thermal cycling conditions.