In humans, sex steroids have been implicated in the regulation of hepatic and lipoprotein lipase activity. Therefore, the effects of orchidectomy and subsequent androgen or estrogen administration on hepatic lipase (HL) and adipose tissue and heart lipoprotein lipase (LPL) were examined. Relative to intact controls, orchidectomy of male rats resulted in no significant change in HL activity and mRNA, or in heart and adipose tissue LPL activity and mRNA levels. Subsequently, a subcutaneous silastic tubing, delivering either testosterone, dihydrotestosterone, nandrolone, or 17 beta-estradiol, was implanted for 5 weeks. All substitution treatments had a tendency to reduce HL activity and to induce HL mRNA levels. This effect was, however, only significant for testosterone which resulted in a decrease in HL activity (238 +/- 15 vs. 328 +/- 31 mU/g tissue; p vs. control < 0.05) and an increase in HL mRNA (166 +/- 11 vs. 100 RAU; p vs. control < 0.01). No significant effects of androgens on LPL expression either in heart or adipose tissue were observed. Adipose tissue LPL activity (20 +/- vs. 35 +/- 4 mU/g; p vs. control < 0.05) and mRNA (28 +/- 4 vs. 100 RAU; p vs. control < 0.001) levels, but not heart LPL, however, were diminished substantially after 17 alpha-estradiol treatment. In conclusion, rat HL is influenced by testosterone, while adipose tissue, but not heart LPL, is reduced after estrogen administration.