The effect of pain on human psychomotor performance was measured in seven healthy volunteers after an intramuscular injection of vitamin B or saline using a controlled cross-over method. Vitamin B, causing moderate to severe pain, or painless saline was injected into the buttock at a time when the subjects' performance was impaired after an intravenous injection of diazepam (0.3 mg/kg). The subjects' psychomotor performance was tested before and 2, 3, and 4 h after diazepam (before, 15 min, and 1 h 15 min after the vitamin B and saline injections). The effects of the vitamin B injection on the subjects' divided attention, reaction or co-ordination skills or their ability to discriminate the fusion of flickering light did not differ from the corresponding effects of the saline injection. The results suggest that pain as such does not have any major influence on human psychomotor performance.