Propanidid was administered intravenously in a single dose at a standard speed to patients undergoing brief dental extractions. Four groups of twenty females received respectively 5, 7, 9 and 11 mg/kg of propanidid and three groups of twenty male patients received a dose of 7, 9 or 11 mg/kg. The time from commencing injection to complete awakening and to full recovery were measured in each patient. It was found that each increase in dosage produced a significant increase in the mean duration of sleep but not in time of full recovery. There was, however, a progressive lengthening of recovery time and a significant difference between recovery following the smallest and largest doses. It was also found that the same dose in mg/kg had a shorter duration of action in males than in females. Finally, comparison of the results for patients aged over 40 years with those in patients under 30 years showed that each dosage produced a longer action in the older age group.