AbstractThe serotonergic probe clomipramine (CMI) was used to measure the degree of activity of the serotonin system in pathological gamblers versus matched controls. To this end, intravenous CMI (12·5 mg) was given and prolactin (Prl), growth hormone (GH) and cortisol (C) levels were measured. Compared to controls, pathological gamblers show a blunted prolactin response, but no effects on GH and C levels. It is concluded that the 5‐HT system shows hypoactivity in pathological gamblers, and this finding supports the inclusion of pathological gamblers in the group of behavioural disturbances characterized by poor impulse contr