Studies have shown abnormal pituitary hormone responses to neuroendocrine agonists in autistic subjects. Two probes (clonidine and L-Dopa) were used to investigate neuroendocrine responses through changes in growth hormone levels. Seven medication-free autistic subjects (ages 6.6 to 19.1) were evaluated and compared to 14 normal controls. Growth hormone was collected at 30-min intervals during the entire study. Clonidine was administered first (dose: 0.15 mgm2), and samples were collected for 180 min. L-Dopa was then administered (dose: 250 mg for subjects <70 lb and 500 mg for subjects >70 lb), and samples were collected for 120 min. There was no difference in the amplitude of the clonidine or L-Dopa peak growth hormone responses in the control versus the autistic subjects. In the autistic subjects, the L-Dopa-stimulated growth hormone peak was delayed and the clonidine growth hormone peak was premature. A statistical difference with the control subjects was found when consideration was given to both the premature response of growth hormone to clonidine and the delayed response to L-Dopa (p=.01, Fisher's Exact Test). These findings suggest possible abnormalities of both dopaminergic and noradrenergic neurotransmission in subjects with autism.