1. Elimination rates of phenylbutazone and warfarin after single oral doses in human volunteers, and steady-state plasma concentrations of these drugs in patients have been measured before, during and after administration of allopurinol.2. Allopurinol, in usual clinical doses, had no significant influence on the elimination rate of either drug in the group of volunteers as a whole although in a few individuals an apparent inhibitory effect was observed.3. No significant changes in the steady-state plasma concentrations of either phenylbutazone or warfarin were observed in patients during the administration of allopurinol.4. It is concluded that although allopurinol may have an apparent inhibitory effect on drug elimination in a few individuals, its administration to most patients on anticoagulant therapy is unlikely to alter dose requirements.