Leathers drying and/or conditioning plant comprising: - an operating tunnel which mainly develops along a linear direction and is provided with air conditioning devices suitable to operate on the leathers in such a way as to dry and/or condition them according to pre-defined treatment cycles; - a plurality of support frames (2; 52; 102), each of which is suitable to receive on at least one of the side faces (2a, 2b) at least one of said leathers in order to keep it in the spread position and formed by two half-frames or portions (6, 7) spaced apart and partially coupled each other so that a first half-frame or portion is movable with respect to a fixed second half-frame or portion; - actuation means operatively connected with said support frames (2; 52; 102) and suitable to be operated in such a way as to extend said leathers to a nominal tension value set by the operator, by opening or spacing said half-frames or portions (6, 7); - moving means, operatively connected with said support frames (2; 52; 102) in order to convey them along said linear direction according to a predetermined speed within said operating tunnel, in order to allow drying and/or conditioning of said leathers, and outside said operating tunnel in order to allow discharge of said leathers dried and/or conditioned in a processing cycle and load of said leathers to be dried and/or conditioned in the next processing cycle; - detection means, operatively connected with said actuation means, whereby said actuation means and said detection means are incorporated in elastically yielding or elastic means (5; 55; 105); - adjustment means (4; 54; 104), operatively associated with said actuation means and cooperating with said actuation means to keep said leathers tensioned substantially to said nominal tension value during said processing cycle, characterized in that said adjustment means (4; 54; 104) are coupled with each of said support frames (2; 52; 102) and are of mechanical and self-adjusting type, cooperating with said elastically yielding or elastic means (5; 55; 105), coupled with each of said support frames (2; 52; 102), in such a way as to obtain a spontaneous continuous self-adjustment of said first movable portion with respect to said second fixed portion of said support frames (2; 52; 102) and, thus, of the actual tension value which each of said leathers secured on said support frames (2; 52; 102) is subjected to during said processing cycle of said leathers keeping said actual tension value of said leathers constantly to said nominal tension value during said processing cycle, wherein said elastically yielding or elastic means (5; 55; 105) are provided with a preload force set by the operator according to the type of said leathers temporarily secured to said support frames (2; 52; 102). - an operating tunnel which mainly develops along a linear direction and is provided with air conditioning devices suitable to operate on the leathers in such a way as to dry and/or condition them according to pre-defined treatment cycles; - a plurality of support frames (2; 52; 102), each of which is suitable to receive on at least one of the side faces (2a, 2b) at least one of said leathers in order to keep it in the spread position and formed by two half-frames or portions (6, 7) spaced apart and partially coupled each other so that a first half-frame or portion is movable with respect to a fixed second half-frame or portion; - actuation means operatively connected with said support frames (2; 52; 102) and suitable to be operated in such a way as to extend said leathers to a nominal tension value set by the operator, by opening or spacing said half-frames or portions (6, 7); - moving means, operatively connected with said support frames (2; 52; 102) in order to convey them along said linear direction according to a predetermined speed within said operating tunnel, in order to allow drying and/or conditioning of said leathers, and outside said operating tunnel in order to allow discharge of said leathers dried and/or conditioned in a processing cycle and load of said leathers to be dried and/or conditioned in the next processing cycle; - detection means, operatively connected with said actuation means, whereby said actuation means and said detection means are incorporated in elastically yielding or elastic means (5; 55; 105); - adjustment means (4; 54; 104), operatively associated with said actuation means and cooperating with said actuation means to keep said leathers tensioned substantially to said nominal tension value during said processing cycle,