The analgesic efficacy and safety of intramuscular meptazinol and pethidine in the first stage of labour were compared in a randomized double-blind trial in 358 patients. Pain relief was measured on a verbal rating scale, maternal side effects were recorded and neonatal outcome assessed in the first 24 h. Pain relief during the first hour after injection was significantly greater in the meptazinol than in the pethidine group at 45 and 60 min. Thereafter, there was no difference between the treatments, and the duration of action was approximately the same. Twenty-eight per cent of patients experienced side effects after meptazinol compared with 35% after pethidine. The commonest were nausea and vomiting with a similar incidence in both groups. Most of the neonatal observations revealed no difference between the two drugs, but significantly more babies whose mothers had received meptazinol had an Apgar score of greater than or equal to 8 at 1 min after birth.