Plasma concentrations of L(+)pseudoephedrine administered in clinically used dosages were determined by gas liquid chromatography using a nitrogen sensitive detector. They were measured after administered of an immediate release formulation (Sudafed) given in either a single dose of 180 mg, or three divided doses of 60 mg, and also after administration of two different sustained release preparations containing 180 mg. Ten subjects each received five treatment regimes, administration being ordered in a balanced design based on 2 five sided Latin squares. Significant differences were found between plasma concentrations and rates of urinary excretion of L(+)pseudoephedrine following administration of the different preparations. Peak plasma concentrations were greatest after 180 mg of the immediate release preparation while more sustained elevations of concentration followed administration of both sustained release preparations and the immediate release preparation in repeated doses. Despite these differences in plasma concentration significant differences in heart rate, blood pressure, or subjective ratings of mental state rarely occured, and the reasons for this are discussed. In a second study, one of the sustained release preparations was administered to 10 subjects at a dose of 180 mg twice daily for two weeks, and plasma concentrations and effects were measured. L(+)pseudoephedrine plasma levels reached a plateau in 3 days producing increased heart rate initially insomnia occurred but this disappeared after 3 days.