The pharmacokinetic behaviour of pemoline was studied in 28 children, aged 5 to 12 years, diagnosed as having the attention deficit disorder with hyperactivity. The mean elimination half-life of pemoline in these children was approximately 7 hours, which is considerably shorter than the half-life of 11 to 13 hours previously reported in adults. The tendency of the half-life to increase with age may be explained by the statistically significant decrease in total body clearance with age. The increasing half-life of pemoline with age should be considered during long term drug therapy. In this study no tolerance to the beneficial effects of pemoline was observed over 6 months.The apparent therapeutic serum concentration range for these children was attained after doses of 37.5 to 131.25mg pemoline daily. Since the optimum serum concentration shows wide variation, the dosing regimen must be determined individually. Routine monitoring of the pemoline serum concentrations is not useful because of this apparent variation in optimum serum concentration and because of the linear relationship between dose and concentration.