The current study was undertaken to determine the effects of sex steroid hormones (estrogen, testosterone and 19-nortestosterone) on proliferation and differentiation of preadipocytes of female and male pigs. The preadipocytes were isolated from the backfat of new-born female and male pigs by collagenase digestion and cultured in the $CO_2$ incubator. The concentration of $10^{-7}M$ and 10-6M sex steroid hormones were treated to the cultured preadipocytes. Regarding the effects on preadipocytes proliferation, high concentration ($10^{-6}M$) of all the three hormones increased proliferation of female preadipocytes,and only estrogen and testosterone increased proliferation of male preadipocytes. Regarding the effects on preadipocyte differentiation, all the three hormones increased differentiation of pig preadipocytes, regardless of hormone concentrations and sex of preadipocytes. The degree of stimulation of cell differentiation by sex steroid hormones was greater than that of cell proliferation.