Direct observation of blink rate was used as a noninvasive, in vivo estimate of dopamine function in adults with mental retardation and repetitive behavior disorders. Blink rate as measured in groups of stereotypy, compulsion, and control subjects was highly stable. Subjects with stereotypies had significantly lower blink rates than did control subjects. Although blink rates for compulsive subjects were not significantly different from those of control subjects, a subgroup of compulsive subjects with comorbid stereotypic behaviors displayed significantly lower blink rates. Significant inverse correlations were found for blink rate and severity of repetitive behavior disorder and for blink rate and ratings of motor slowness. These findings support the hypothesis that stereotyped behavior among adults with mental retardation is mediated by hypodopaminergic function.