The time course of antipsychotic response following the initiation of an antipsychotic drug and functional dopamine receptor sensitivity were explored in a cohort of recently admitted psychotic (mood-incongruent) patients. The distribution of the latencies of antipsychotic response suggested at least two populations. Rapid responders (RRs) had 60% reduction of baseline psychotic symptoms by a mean of 5.5 days of drug treatment. Delayed/nonresponders required 2-7 weeks for a similar reduction of psychotic symptoms. The sensitivity of postsynaptic dopamine receptors was explored using a neuroendocrine probe: growth hormone response to the dopamine agonist, apomorphine (AP). RRs had an exaggerated growth hormone response to AP in comparison to delayed/nonresponders (P less than 0.05). Exaggerated sensitivity of postsynaptic dopamine receptors and rapid antipsychotic response following dopamine receptor blockade in RRs suggest a true functional dopamine hypersensitivity disorder in the RR group. In contrast, lower postsynaptic receptor sensitivity (as reflected by lower growth hormone response to AP) and failure of early response following dopamine receptor blockade focus attention away from dopamine hyperactivity as a relevant etiologic mechanism in delayed/nonresponders. Response rates to neuroleptic drugs and neuroendocrine probes of receptor sensitivity may separate two or more etiologically distinct diseases with schizophrenic-like symptoms.