Changes in glomerular filtration rate and effective renal plasma flow following a protein meal were measured in seven patients with essential hypertension on no treatment and after 6 weeks' treatment with nifedipine and mefruside. Glomerular filtration rate and effective renal plasma flow increased significantly following a protein meal in patients on no treatment (P less than 0.01 and P less than 0.05 respectively). The response to a protein meal was lost following antihypertensive treatment (P less than 0.5 and P less than 0.1 respectively). Although there was some increase in the fasting values of glomerular filtration rate and effective renal plasma flow this was less pronounced and more difficult to demonstrate when only fasting values were compared. We propose that the loss of response to a protein meal is due to recruitment of renal reserve function and that protein meal challenge is a sensitive test for detecting changes in renal function.