Growth hormone (GH) regulation of serum lipoproteins and apolipoproteins was studied using hypophysectomized (Hx) male and female Sprague-Dawley rats. Hypophysectomies were performed at 45 or 50 days of age. Hx rats were given replacement therapy with L-thyroxine (10 micrograms/kg/d) and hydrocortisone (400 micrograms/kg/d) unless otherwise specified. Bovine GH (bGH) was given either as two daily subcutaneous (SC) injections at 12-hour intervals or as a continuous SC infusion. Serum cholesterol and apolipoprotein (apo) E concentrations decreased after Hx of female rats. In contrast, Hx of male rats resulted in increased serum cholesterol concentrations and had no effect on serum apo E concentrations. There were no effects of Hx on high-density lipoprotein (HDL) apo E levels in male rats in contrast to female rats. bGH given twice daily to Hx male rats had no effect on HDL apo E levels, but a continuous infusion of bGH resulted in a marked increase in HDL apo E concentration, to levels above those of intact male rats. As previously observed in female rats, serum and HDL apo A-I concentrations decreased and serum and low-density lipoprotein (LDL) concentrations of apo B increased after Hx of male rats. Treatment with L-thyroxine and hydrocortisone reduced the serum concentrations of apo B. bGH given alone resulted in even lower concentrations of apo B. Serum concentrations of cholesterol and apo E were unaffected by replacement therapy with L-thyroxine and hydrocortisone. Treatment with bGH alone had similar effects on serum cholesterol, apo E, and apo B concentrations as treatment with L-thyroxine, hydrocortisone, and bGH in combination.(ABSTRACT TRUNCATED AT 250 WORDS)