Procaine esterase activity in plasma from patients with renal failure is decreased by 40%. Since cyanate is formed from urea and readily carbamylates certain blood proteins, a possible role for cyanate in the depression of plasma esterase activity in uraemic patients was considered. However, in vitro carbamylation of normal plasma in a range similar to that detected in uraemic patients did not influence procaine esterase activity. Kinetic analysis of the reaction showed that the maximal hydrolyzing capacity but not the Km in uraemic plasma was diminished (5.0 +/- 0.3 X 10(-5) moles hydrolyzed per litre of plasma per minute and a Km of 3.9 +/- 0.2 X 10(-5) mol/l in plasma from normal volunteers as compared to 3.1 +/- 0.1 X 10(-5) mol/l/min and 3.5 +/- 0.2 X 10(-5) ml/l in plasma from patients with renal failure). Therefore, not carbamylation but rather a decrease in enzyme synthesis is the likely explanation for the lower rate of procaine hydrolysis in uraemic plasma.