The authors sought to determine the analgesic activity of clomipramine (CMP) versus placebo, efficacy and side-effects according to the time of administration, long term clinical results and the relationship between plasma levels and analgesic effect. This double-blind randomised trial involved the infusion of 250 ml of glucose solution morning and evening versus placebo. CMP was administered at progressive doses for 8 days (maximum dosage 75 mg), either in the morning at 8 a.m. or in the evening at 6 p.m. Sixty eight patients accepted to be treated, all suffering from low back pain with or without sciatica. Maintenance treatment at the dose of 75 mg/day was then administered. CMP had a statistically significant analgesic action independent (rapidity of action) of its antidepressant activity. CMP was more effective and better tolerated (sedative and tranquilizer effect) in the evening. Long term results were poor (75% of cases). There was no link between plasma levels and clinical response. The authors noted that the analgesic activity of CMP was rapid, but for a duration limited to the short term and that this efficacy compared with the quite good results obtained with the placebo could be explained by a possible methodological bias (patients hospitalised and treated by intravenous infusions). The finding that CMP was better tolerated in the evening, without any loss of efficacy, is a positive feature (chronotherapeutic trial). Poor long term results could be partially explained by side-effects (66%). There was no relationship between analgesic effect and plasma levels.