Leptin has been demonstrated to induce adipose tissue apoptosis, which can contribute to the decrease of adiposity, after either central nervous system or peripheral administration. However, it is not known whether leptin acts only centrally to initiate a signal or can also act directly on adipocytes to induce apoptosis. The objective of this study was to determine the direct effect of leptin on adipocyte apoptosis and adipogenesis in vitro using 3T3-L1 cell lines. An ELISA for single stranded DNA, which is highly specific for apoptotic cells, was used to quantify apoptosis. Preconfluent preadipocytes treated with 10(-9), 10(-8), 10(-7), and 10(-6)M leptin showed inhibitory effects on cell viability, and similar observations were also found in maturing preadipocytes treated during day 0-2 and day 2-4 of maturation. After 48 h incubation with 10(-6)M leptin, LDH release was increased by 24.3% (p<0.05) in preconfluent preadipocytes and by 108.5% (p<0.01) in maturing preadipocytes. However, ssDNA analysis revealed no increased apoptosis in preconfluent or maturing preadipocytes or in mature adipocytes treated with leptin. Leptin significantly reduced lipid accumulation and GPDH activity in maturing preadipocytes, demonstrating an inhibitory effect of leptin on adipogenesis. These results indicate that leptin does not act directly to induce adipocyte apoptosis, but can act directly to inhibit maturation of preadipocytes.