The importance of insulin for the in vivo effects of growth hormone (GH) on lipid and lipoprotein metabolism was investigated by examining the effects of GH treatment of hypophysectomized (Hx) female rats with and without concomitant insulin treatment. Hypophysectomy-induced changes of HDL, apolipoprotein (apo)E, LDL, and apoB levels were normalized by GH treatment but not affected by insulin treatment. The hepatic triglyceride secretion rate was lower in Hx rats than in normal rats and increased by GH treatment. This effect of GH was blunted by insulin treatment. The triglyceride content in the liver changed in parallel with the changes in triglyceride secretion rate, indicating that the effect of the hormones on triglyceride secretion was dependent on changed availability of triglycerides for VLDL assembly. GH and insulin independently increased editing of apoB mRNA, but the effects were not additive. The expression of fatty-acid synthase (FAS), stearoyl-CoA desaturase-1 (SCD-1), and sterol regulatory element-binding protein-1c (SREBP-1c) was increased by GH treatment. Insulin and GH had no additive effects on these genes; instead, insulin blunted the effect of GH on SREBP-1c mRNA. In contrast to the liver, adipose tissue expression of SREBP-1c, FAS, or SCD-1 mRNA was not influenced by GH. In conclusion, the increased hepatic expression of lipogenic enzymes after GH treatment may be explained by increased expression of SREBP-1c. Insulin does not mediate the effects of GH but inhibits the stimulatory effect of GH on hepatic SREBP-1c expression and triglyceride secretion rate.