Development of a liquid controlled-release pseudoephedrine [bronchodilator drug] product is described. Two bioequivalence studies were conducted. In a single-dose study involving 20 subjects, the bioavailabilities of 5 controlled-release suspensions with a broad range of drug-release rates were compared with an immediate-release form of pseudoephedrine HCl in a 4-way crossover, incomplete block, sequence-randomized study. Serial blood sampling up to 36 h after drug ingestion provided area-under-the-curve (AUC), maximum plasma concentration (Cmax) and time to peak (tmax). In the multiple-dose study, involving 18 subjects, the bioavailability of the optimal formulation determined from the single-dose study was compared with a reference pseudoephedrine HCl syrup. Serial blood sampling up to 12 h after drug ingestion was performed to determine AUC, Cmax and tmax. The single-dose investigation showed that all formulations were bioequivalent except the product with the slowest release rate, which had lower AUC and Cmax values. The results of the multiple-dose study confirmed these findings with the reference syrup. The use of a series of drug formulations with a wide range of release rates permitted selection of an optimal product in addition to providing the information needed to ensure continuous production of bioequivalent products.