As a further test of the hypothesis of serotonin hypersensitivity in panic disorder (PD), the serotonin agonist meta-chlorophenylpiperazine (MCPP) was administered intravenously in a dose of 0.05 mg/kg to 27 PD patients and 22 normal control subjects. This is one-half the dose used in our previous study of PD patients, where the dose may have been too high to provide evidence of hypersensitivity to the agent. Responses of anxiety and nervousness were statistically indistinguishable by analysis of variance in the two groups, replicating our previous findings. Panic attack symptom score (PASS) ratings were significantly higher in the PD group, compared with a trend toward higher PASS ratings in the 0.1 mg/kg study. Cortisol, human growth hormone, and male prolactin responses showed no significant differences in the two groups by analysis of variance. Prolactin responses were significantly blunted in the female patients. The unexpected blunted prolactin response to MCPP in female PD patients may reflect a nonspecific blunting of prolactin response to stress. The PASS data provide some evidence of serotonergic hypersensitivity in PD.