In studies of the acute effects of haloperidol on the severity of stuttering in 12 subjects not in treatment at the time of drug evaluation, a single 0.5 mg haloperidol injection was found to increase fluency in 9 to 12 subjects, as compared with saline placebo. The average improvement in those subjects who improved was 25% on reading and 40% on spontaneous speech. Side effects from this dose of haloperidol were minimal. The effects of apomorphine on speech were not statistically significant, but increased fluency was seen in a number of subjects on the reading test. The results of this study suggest that acute drug evaluation studies may be valuable in determining the effects of various psychotropic agents on the severity of stuttering. The increased fluency after haloperidol, an agent which is felt to turn off the dopaminergic system via postsynaptic blockade, and after low-dose apomorphine, which appears to inhibit the dopamine system via presynaptic effects is consistent with a role for central dopaminergic systems in the pathogenesis of stuttering. Further studies are needed to confirm this hypothesis.