The present study was conducted to determine whether corticosteroids influence the inductive effect of growth hormone (GH) on plasma concentrations of insulin-like growth factor I (IGF-I). The first experiment was designed to determine the effects of corticosterone alone on basal concentrations of IGF-I. Rats were treated daily for 4 d with 0, 50, 100, 250, or 500 mg of corticosterone/kg of BW. There was a close positive relationship between the dose of steroid injected and plasma concentrations of corticosterone and a close negative relationship between plasma corticosterone and growth. Plasma concentrations of IGF-I showed a positive relationship to dose and plasma concentrations of corticosterone and a negative relationship to growth rate. In the second experiment, rats were treated daily for 21 d with either porcine growth hormone (10 mg of pGH/kg of BW), pGH plus corticosteroid, or vehicle. The dose of steroid administered was increased every 3 d until the mean weight gain of the group was zero. Animals treated with pGH alone gained significantly more weight than controls. This growth response was not impaired significantly by corticosterone acetate at doses up to 500 mg/kg of BW. The more potent corticosteroid, cortisone, arrested the growth of pGH-treated rats at a dose of 80 mg/kg of BW, however. Plasma concentrations of IGF-I were increased by pGH treatment (57%) and increased further by concurrent cortisone treatment (212%). In summary, corticosteroids increase plasma concentrations of IGF-I and enhance the inductive effect of pGH on this hormone despite their catabolic actions.