A high protein meal acutely stimulates the glomerular filtration rate. In order to assess the possible role of atrial natriuretic peptide (ANP) as a mediator of this renal response, the ANP plasma concentrations were measured with a sensitive radioimmunoassay method before and after a large protein load (1.5 g/kg body weight given as a meat meal). Creatinine clearance increased significantly while fractional excretion of sodium remained constant. ANP plasma concentrations did not change after the protein load and the intra-individual variations of creatinine clearance were not correlated with the intra-individual changes in ANP plasma concentrations. It was concluded that ANP was probably not involved in the renal haemodynamic changes observed after a large protein load.