Objective: This double-blind study was undertaken to compare central serotonergic function in depressed patients and healthy comparison subjects by examining neuroendocrine and mood responses to intravenous infusion of the serotonin agonist m-chlorophenylpiperazine (mCPP). Method: The participants were 20 drug-free patients with DSM-III-R major depression and 18 healthy comparison subjects. After an overnight fast, the subjects received an intravenous infusion of mCPP, 0.1 mg/kg, or placebo saline. Blood was obtained for measurement of serum prolactin, growth hormone (GH), and cortisol. Visual analogue scales were used to assess mood. Results: The depressed patients had a blunted GH response and felt less drowsy than the comparison subjects; prolactin, cortisol, and the remaining behavioral ratings showed no differences between the two groups. Conclusions: In light of findings with other provocative agents, the blunted GH response to mCPP may reflect a defect in GH production in depression and could be a marker of the depressed state. The lack of differences in the other neuroendocrine variables suggests that the functioning of postsynaptic serotonergic receptors responsive to mCPP may be relatively intact in depression.