The sympathetic nervous system was evaluated in 11 primary autistic patients and their families. The plasma levels of norepinephrine (NE), the neurotransmitter of the sympathetic nervous system, was higher in the patients than in age-controlled normal volunteers both while supine and after standing. The plasma activity of dopamine-beta-hydroxylase, the enzyme that converts dopamine to NE, was significantly lower in the autistic patients and their healthy relatives than in control groups. Dopamine-beta-hydroxylase does not appear to provide an index of sympathetic activity in this group of patients who, on the basis of the elevated plasma levels of NE, may demonstrate a chronic state of hyperactivity of the sympathetic nervous system. Low enzyme activity found in both the autistic patients and their immediate families may be associated with this disorder.