1 Plasma and milk concentrations of pseudoephedrine and triprolidine were determined (by radioimmunoassay) in three lactating mothers over 12-48 h after ingestion of a combination medication containing 60 mg of pseudoephedrine hydrochloride and 2.5 mg of triprolidine hydrochloride monohydrate. J 2 Pseudoephedrine concentrations in milk were consistently higher than those in plasma. The total amount of drug in milk, as judged by areas under the respective curves (AUC), was two to three times greater than in plasma. 3 Triprolidine concentrations in milk and plasma were more variable between subjects than those of pseudoephedrine. AUC values for milk and plasma were similar for one subject, while the plasma value exceeded that for milk in another woman. 4 The fraction of the dose excreted in milk was estimated to be 0.4-0.7% for pseudoephedrine and 0.06-0.2% for triprolidine.