The effects of dexmedetomidine 1.0 microgram.kg-1, midazolam 20 micrograms.kg-1 and saline placebo were assessed in a double-blind, randomised study in 90 patients undergoing day-case cataract surgery under regional anaesthesia. The trial drug was injected into the deltoid muscle 45 min before the peri-ocular block. Dexmedetomidine 1.0 microgram.kg-1 decreased intra-ocular pressure before, during and after surgery. The maximum reduction in mean (SD) intra-ocular pressure occurred in the dexmedetomine group just before discharge from hospital (17.7 (2.8) mmHg to 11.5 (2.9) mmHg) (p < 0.001 compared with midazolam and placebo). In contrast, midazolam did not differ from saline placebo. Dexmedetomidine and midazolam produced a similar sedative effect of short duration. Dexmedetomidine induced a moderate decrease in blood pressure (p < 0.001 compared with placebo) and a slight but statistically significant decrease in heart rate throughout the study period (p < 0.001 compared with placebo). Dexmedetomidine 1.0 microgram.kg-1 intramuscularly, effectively reduced intra-ocular pressure and produced short-acting sedation with marginal cardiovascular effects; it may be a useful premedicant drug for elderly patients undergoing day-case cataract surgery under regional anaesthesia.