The current study was undertaken to determine the effect of retinoic acid (RA) on proliferation and differentiation of preadipocytes from male and female pigs. The preadipocytes were isolated from new-born male and female pigs by collagenase digestion and washed three times one day after seeding (designated as day 0 of culture). RA was included in the media at various concentratives from day 0 to 2. The cell number was measured on day 2 with hematocytometer after trypsin digestion. Cell differentiation was determined on day 6 by measuring glycerol-3-phosphate dehydrogenase activity. RA (0.1, 1 and 10 uM) showed no effect on proliferation of preadipocytes from both male and female pigs. However, RA significantly decreased differentiation of pig preadipocytes. Degree of differentiation with 0.1 uM, 1 uM and 10 uM of RA treatment was 80%, 41% and 29% respectively, compared with control. Similar inhibitory effect was found in the female pigs; 77%, 28% and 16% respectively. It is interesting that RA treated on cell proliferation stage had no effect on proliferation but had a strong inhibitory effect on differentiation which is happening in the late stage of cell culture.