The clinical decision to use a particular antidepressant should be made with reference to the behavioural toxicity profiles of substances in current use. Antidepressants can be cardiotoxic, proconvulsant, able to cause weight gain and sleep disturbance, and also impair psychological functions necessary for everyday living. Behavioural toxicity (reduction in psychomotor activity or cognitive ability) tends to augment levels of psychomotor and cognitive retardation; meta-analysis of controlled studies of antidepressants shows that some tricyclics can disrupt these functions. In comparison with these, moclobemide is relatively free from significant behavioural toxicity within the dose-ranges used. No relevant differences were found between placebo and 200 mg moclobemide on a battery of psychomotor and cognitive tests: with 400 mg, there was a significant impairment of peripheral reaction time, but no other measure of the test battery was impaired. In comparison, amitriptyline 50 mg, produced a noticeable and significant impairment of psychomotor and cognitive skills on most test measures. On the whole moclobemide has been found to be free from any behavioural toxicity likely to interfere with the well-being of patients or their performance of the tasks of everyday living.