GH has previously been shown to regulate serum lipoprotein levels and hepatic secretion of apolipoprotein-B (apo-B) and apo-E in the rat. The aim of this investigation was to study a possible role of insulin-like growth factor-I (IGF-I) in this regulation. Adult female rats were hypophysectomized and treated with recombinant human IGF-I (1.25 mg/kg.day) as a sc continuous infusion for 7 days. The effects of IGF-I were compared with those of bovine GH, given either as a continuous sc infusion or as two daily sc injections. All hypophysectomized rats were given replacement therapy with L-T4 and cortisol. Serum IGF-I concentrations increased to similar levels as a result of treatment with bovine GH and IGF-I. There was no effect of IGF-I on serum concentrations of glucose or insulin, whereas GH, independent of its mode of administration, increased serum insulin concentrations. Food intake was not affected by treatment with IGF-I. IGF-I had no effect on serum concentrations of cholesterol or apo-E, whereas GH given twice daily decreased serum cholesterol concentrations, and a continuous infusion of GH increased serum apo-E concentrations. Serum triglyceride and apo-B concentrations increased markedly as a result of IGF-I treatment, whereas GH had no effect on serum triglycerides, but decreased serum apo-B concentrations. Hepatocytes were isolated from hypophysectomized rats treated with L-T4 and cortisol alone or in combination with IGF-I and kept in short term cultures. In this system, IGF-I had no effect on the incorporation of [3H]glycerol in triglycerides or the mass of triglycerides in the cells and medium. There was no effect of IGF-I treatment on the secretion of apo-E or apo-B. Moreover, there was no effect of IGF-I treatment on the relationship between newly synthesized and secreted apo-B 48 and apo-B 100, as determined by [35S]methionine labeling of the proteins. In conclusion, the previously observed effects of GH on serum lipoproteins and hepatic apolipoprotein secretion does not seem to be mediated via IGF-I, but IGF-I has its own unique effects on serum triglyceride and apo-B levels. The increases in serum apo-B and serum triglyceride concentrations after IGF-I treatment were not dependent on increased hepatic secretion of apo-B or triglycerides.