Propylthiouracil (PTU) disposition was investigated in hyperthyroid women during pregnancy and post partum. The plasma half-life after the first dose averaged 0.66 h and became greater than 1 h with chronic drug therapy. The apparent volume of distribution after the first dose averaged 0.26 liter/kg, similar to nonpregnant hyperthyroid patients. There was a tendency to increased apparent volume of distribution with chronic drug ingestion. Plasma clearance of PTU showed no change related to disease or to pregnancy. Our data suggest that pregnancy does not have a major effect on the pharmacokinetic disposition of PTU in the hyperthyroid mother. Observation of a prolonged plasma half-life for PTU in 1 patient post partum deserves further investigation to determine if this change is associated with clinically significant therapeutic or toxic effects.