After a single intravenous dose of 6 mg serum concentrations of pancuronium were measured in 6 anuric patients by means of the bromophenol blue colorimetric method at 1, 5, 45 min and 3 h. After 45 min and 3 h there was a retention of 1.7 and 4 times the control values respectively. The prolongation of pancuronium induced muscular paralysis which has been demonstrated by other authors now can definitely be related to elevated serum concentrations of the active drug. The termination of muscle relaxation in anuric patients is considered merely to be due to redistribution of the drug from the motor end plates to non specific structures. Prolonged neuromuscular blockade may occur in as much as the redistribution mechanism is overloaded by inadequate doses.