The prolactin response to dl-fenfluramine (an indirect central serotonin agonist) challenge has been used to assess serotonergic function and appears to be blunted in depressed patients. We used this method to determine whether the serotonergic deficit in depressed patients is corrected by treatment with paroxetine. Prior to treatment with paroxetine sixteen depressed patients received a placebo challenge followed by a dl-fenfluramine challenge the next day. The same two challenges were repeated after treatment. Prolactin baseline levels were measured before pill administration, and then hourly for 5 hours. Fenfluramine/norfenfluramine levels were assayed at each time point after drug administration. Treatment with paroxetine significantly increased the baseline prolactin level independently of treatment response but positively correlated with paroxetine dose. We found that pre-treatment prolactin response to dl-fenfluramine challenge did not predict clinical response to paroxetine, nor did the prolactin response change significantly after treatment. There was no significant difference in the post-treatment prolactin response between treatment responders and treatment non-responders. We found evidence of increased prolactin levels that may reflect effects of paroxetine in enhancing serotonin levels. Acute release of serotonin as measured by the prolactin response to fenfluramine is not altered by paroxetine treatment.