The aim of this double-blind, placebo-controlled study was to evaluate the effect of intravenously administered morphine in humans using an argon laser to induce experimental pain. Thirty volunteers were randomised to receive a total of 0.15 mg.kg-1 morphine intravenously or saline. The argon laser was used to determine the possible change in warmth thresholds and pin prick pain thresholds. Measurements were performed before and at 10, 20, 30 and 40 min after the injection. During the observation period the warmth thresholds increased 20.7% (P < or = 0.05) from baseline (0.82 W +/- 0.42 W) in the morphine group while an increase of 14.3% (P > or = 0.05) was seen in the placebo group (Baseline: 0.91 +/- 0.37). The pinprick pain thresholds of the morphine group increased 9.4% (P < or = 0.05) from baseline (1.39 W +/- 0.7 W) while the corresponding thresholds of the placebo group was 4.6% (P > or = 0.05) (baseline: 1.73 W +/- 0.44 W). The differences seen between the morphine group and the placebo group were not statistically significant and thus it was demonstrated that morphine had no effect of either the feeling of warmth or the pain elicited by the argon laser. The present study supports other investigations and clinical experience suggesting that intermittent pain is not relieved by morphine unlike continuous pain, which can be relieved by morphine.