In several reports, the acute oral administration of the partial serotonergic agonist meta-chlorophenylpiperazine (mCPP) in dose of 0. 5 mg/kg induced a significant worsening of obsessive-compulsive (OC) symptoms in a number of patients. The aim of our study was to test the 0.25 mg/kg mCPP dose, which was hypothesized to be more specific for OC symptoms and was until now tested only on healthy subjects. In a double-blind, controlled crossover study, 12 OC patients participated on three test days, receiving one of the following on each day: oral 0.5 mg/kg mCPP (standard dose), 0.25 mg/kg mCPP (low dose), or placebo. Behavioral ratings were obtained by means of Visual Analogue Scale (VAS) ratings. The low dose mCPP induced a significant worsening of OC symptoms in 50% (6/12) of the patients, whereas 8.3% (1/12) of the patients showed a worsening after the standard dose. On the other hand, only the standard dose mCPP induced a worsening, although not statistically significant, of anxiety ratings. Our data show that the 0.25 mg/kg dose mCPP induces a specific response in OC symptoms, with little anxiogenic effect. To confirm these preliminary data, future studies will be needed on larger samples and with more sensitive rating scales.