Six healthy females who had been nursing their infants for 6 to 11 months received a single, 200-mg oral dose of suprofen, an analgesic which has been evaluated clinically. Blood and milk samples were collected at discrete times over an 8-hour period and suprofen concentrations in milk and plasma were determined by HPLC. The binding of suprofen to milk and plasma proteins was determined by equilibrium dialysis. The maximum concentrations of suprofen in the milk ranged from 0.118 to 0.232 microgram/ml and occurred from 1 to 2 hours after dose administration. The maximum plasma suprofen concentrations ranged from 13.8 to 28.3 micrograms/ml and occurred from 0.5 to 2 hours after dosing. Within any subject, the peak suprofen concentration in milk was 0.5 to 0.9 per cent of the peak concentration in plasma. Suprofen was extensively bound to plasma proteins (99.4 per cent) and minimally bound to milk proteins (10 per cent). The average milk/plasma ratio based on area-under-the-curve measurements was approximately 0.014, or 1.4 per cent. This ratio agrees well with an estimated value of 1.2 per cent for the pH-dependent, passive diffusion of suprofen from plasma into milk. From these data, it appears that there would be minimal suprofen exposure to a nursing infant after administration of recommended doses to the nursing mother.