Rats subcutaneously implanted with AH109A hepatoma cells show hyperlipidemia with high concentrations of serum triglyceride and nonesterified fatty acid, suppression of lipoprotein lipase (LPL), and elevation of hormone-sensitive lipase (HSL) activities during the growth of the hepatoma. Supplementation of the diet with sulfur amino acids such as L: -methionine (Met) and L: -cystine (Cys) improved hyperlipidemia by restoring LPL and HSL activities. In the present study, we have attempted to examine the effects of sulfur amino acids on the activity and mRNA level of LPL and the activity of HSL using 3T3-L1 cells, which are known to differentiate to adipocytes. The adipocytes were incubated with various concentrations of Met, Cys or L: -cysteine (CysH) in the absence or presence of tumor necrosis factor-alpha (TNF-alpha). LPL activity was suppressed by TNF-alpha. In the absence of TNF-alpha, Met, Cys and CysH did not change the LPL activity. In the presence of TNF-alpha, Met and Cys significantly increased the LPL activity, and Met also enhanced the LPL mRNA level. HSL activity was also suppressed by TNF-alpha. In the absence of TNF-alpha, Met enhanced the HSL activity. In the presence of TNF-alpha, Met, Cys and CysH suppressed the HSL activity. Sulfur amino acids such as Met, Cys and CysH affected the LPL activity, mRNA level, and HSL activity in 3T3-L1 adipocytes. Some of these effects of sulfur amino acids were different between LPL and HSL, between the absence and the presence of TNF-alpha, and between 3T3-L1 adipocytes and the adipose tissue from rats.