This study was undertaken to test the reliability of a sustained-release dosage form of dextroamphetamine sulfate. We tagged the drug with the radioactive carbon atom and gave four dosage regimens to 16 subjects following an incomplete block crossover design. The plasma and urine data obtained showed that the 15-mg sustained-release dosage form tested performed in a way comparable to 5 mg given three times each day, that it released its drug over a prolonged period, and that it was equivalent to other 15-mg regimens in the amount of drug made available for absorption. On the basis of these findings, we can say that the sustainedrelease dosage form tested is a reliable way to administer dextroamphetamine sulfate. The tests described herein, and tests of similar complexity, are becoming a more and more essential part of the evaluation of such dosage forms.