The temporal relationships between the changes in inulin and p-aminohippurate clearances and plasma growth hormone (GH) and insulin-like growth factor I (IGF I) levels were examined in a man with hypothalamic GH deficiency before and during the first 6 days of treatment with daily GH injections. The patient ate a diet with a constant protein and salt content from 1 week before the study until it was completed. During the 4-hour period immediately after the first GH injection, plasma GH rose markedly, but plasma IGF I was not detectable, and effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) did not change from baseline. On the next day, before the second GH injection was given, plasma GH was only slightly elevated, plasma IGF I had increased, and ERPF and GFR had risen by +35.5 ± 2.1 % (SEM) and +22.7 ± 2.8%, respectively. On the 4th and 7th days, immediately before the GH injections, there was no further rise in ERPF and GFR, both of which remained well above baseline values. At these times, plasma GH levels were at baseline, but plasma IGF I continued to rise progressively. These data are consistent with the thesis that the low ERPF and GFR in GH deficiency is due to the lack of synthesis of IGF I rather than the deficiency in GH per se. The data are also consistent with a stimulatory effect of IGF I on ERPF and GFR.