To evaluate the glomerular filtration rate (GFR) response to a protein meal in patients with diabetes and to study the role of glucagon and growth hormone, we studied inulin clearance for three 30-minute periods before and 3 hours after an 80 g protein meal in seven healthy volunteers and 10 patients with diabetes. Patients with diabetes were chosen because their renal response to such a meal has been reported to be abnormal. All had an increase in GFR and plasma glucagon levels after the protein meal. The peak rise in GFR occurred from 1 to 2 1/2 hours after the meal (mean +/- SEM, delta 26 +/- 5 mL/min/m2, controls; delta 22 +/- 7 mL/min/m2, patients with diabetes), with the mean time to normal rise in GFR occurring at 2 hours after the meal. Similarly, plasma glucagon values peaked at different times in individual patients (delta 769 +/- 532 pg/mL, controls; delta 267 +/- 69 pg/mL, patients with diabetes), with the mean plasma glucagon rise occurring 1 hours after the meal. Premeal growth hormone levels tended to be higher in the patients with diabetes (7.6 +/- 1.4 vs 2.1 +/- 0.4 ng/mL), and did not change after the meal. To allow study of the contribution of the increased plasma glucagon to the rise in GFR, eight of these patients (five with diabetes) volunteered to undergo a second GFR response test with a simultaneous infusion of somatostatin. The glucagon response was significantly lowered at all time periods during the infusion (P less than 0.05); no significant change in growth hormone occurred. Without somatostatin in these eight patients, peak increase in postmeal GFR average 20.6 +/- 1.5 mL/min/m2; with the somatostatin, peak increase in GFR was 6.0 +/- 1.8 mL/min/m2 (P less than 0.01). Neither metabolic control nor degree of albuminuria was significantly different at the time of the two studies. Thus, as has been shown in animals, somatostatin infusion limits the rise in GFR after a protein meal in humans.