SUMMARY(1) Plasma and urinary levels of methylguanidine, guanidinosuccinic acid and guanidinoacetic acid were measured in normal subjects and in patients with renal failure.(2) Urinary excretion of methylguanidine and guanidinosuccinic acid was increased in renal failure. Plasma guanidinosuccinic acid concentration was increased and was significantly correlated with the plasma urea concentration. The plasma concentration of methylguanidine was not consistently elevated probably because of an adaptive increase in renal tubular excretion. The excretion of guanidinoacetic acid was decreased in renal failure and plasma concentration remained low.(3) All three guanidines were removed by haemodialysis and their plasma concentrations, although not normal, were much lower in regularly dialysed subjects.(4) Guanidinosuccinic acid and methylguanidine appear to provide alternative pathways for the excretion of nitrogen in patients with renal failure. In advanced uraemia the plasma concentration of methylguanidine and guanidinosuccinic acid may reach levels which have been associated with toxic effects. However, their importance in relation to other uraemic toxins remains difficult to assess.