Seven healthy male volunteers were studied to test the effect of timing of an oral protein load on renal function. Creatinine clearance (Ccr) was measured during the 4-h period after administration of 72 g of protein in the form of cooked red meat at 1200 hours (lunch protein load, PL) and at 2000 hours (supper PL) the next evening. A low-protein meal in the form of vegetables was given as a control load at 2000 hours on the first day (supper control load, CL) and at 1200 hours on the second day (lunch CL). The 12-h night-time Ccr at fasting was used as the baseline reference value. After the lunch PL, Ccr (mean 127 +/- 6.8 ml/min) was 45% (p less than 0.001) higher than the baseline value (mean 87.9 +/- 5.3 ml/min) and 33% (p less than 0.001) higher than lunch CL (mean 95.8 +/- 5.6 ml/min). After the supper PL, Ccr (mean 106.2 +/- 8.7 ml/min) was 20% (p less than 0.01) higher than the baseline value and 15% (p less than 0.01) higher than the supper CL (mean 93.0 +/- 9.3 ml/min). The differences between baseline and control load values were not statistically significant. In all seven patients, the protein load induced a maximum Ccr value at lunchtime, and Ccr after the lunch PL was 22% higher than Ccr after the supper PL (p less than 0.01). We conclude that in healthy individuals, the Ccr after an oral protein load is correlated to the hour of the day when the study is performed.(ABSTRACT TRUNCATED AT 250 WORDS)