The effects of treatment for 11 days with human growth hormone (hGH; 140 micrograms/day), thyroxine (T4; micrograms/day) and hGH+T4 on renal growth and content of insulin-like growth factor-I (IGF-I) in hypophysectomized rats have been compared with saline-treated hypophysectomized animals and intact control animals. Right kidney weight and kidney weight/body weight ratio remained low in the saline-treated group (313 +/- 9 mg vs 694 +/- 28 mg in controls on day 11, P < 0.001 and 3.4 +/- 0.12 x 10(-3) vs 4.2 +/- 0.10 x 10(-3), P < 0.005 respectively). In T4- and hGH-treated animals, kidney weight gain was similar (to 420 +/- 14 and 450 +/- 22 mg on day 11 respectively, P > 0.05), whilst the increase was greater in the group given hGH+T4 (to 572 +/- 34 mg, P < 0.001 compared with hGH- and T4-treated groups). The kidney weight/body weight ratio became normal in the T4- and hGH+T4-treated animals but remained low in the hGH-treated group. The renal content of IGF-I was low in the saline-treated animals throughout the study (92 +/- 10 ng/g on day 11 vs 219 +/- 8 ng/g in control animals, P < 0.001), but increased to a maximum of 88% above baseline on day 1 in the group given T4.(ABSTRACT TRUNCATED AT 250 WORDS)