We have measured the onset and extent of the miotic effect of morphine and alfentanil in conscious patients. Forty unpremedicated ASA I and II patients were allocated randomly to four groups to receive either i.v. saline (control group), morphine 0.1 mg kg-1, alfentanil 4.0 micrograms kg-1 or a combination of these doses, and pupil diameters were measured for the next 30 min. There were no significant differences in the control diameters. In the opioid groups, a significant decrease in diameter (about 1 mm), occurred 4 min after administration of the drug and persisted throughout the study. The opioid groups behaved similarly for 25 min. After 10 min the mean diameter of the alfentanil group began to increase, but this did not reach statistical significance until after 25 min.