The stimulant effect of L-dopa (125 to 500 mg) was compared to dextroamphetamine and methylphenidate, 15, and 20 mg, respectively, on growth hormone secretion in 20 hyperactive children. All three stimulants were responsible for peak GH concentration in serum at 60 minutes after drug ingestion; there was no significant difference between the mean GH level at any time of sampling. Seven of the children were retested with L-dopa and dextroamphetamine after six to eight months of treatment with methylphenidate. After treatment, there was a tendency to higher zero time levels of GH, and to delayed and/or paradoxical response to dextroamphetamine. The findings indicate an acute and a probably long-term effect of dextroamphetamine and methylphenidate on the homeostasis of growth hormone. The possible long-term adverse effects of these drugs on the growth of children indicates the need for caution to the widespread use of these agents.