The effects of pre-anaesthetic medication on target propofol concentration, induction dose, time to induction, and discomfort on infusion were studied in 45 female patients undergoing ambulatory gynaecological procedures using 'Diprifusor' target controlled infusion of propofol. The patients were randomly allocated to receive either no premedication (group 1) or premedication with diazepam alone (group 2) or in combination with alfentanil (group 3). Induction was more successful in premedicated than unpremedicated patients with an initial target propofol concentration of 4 μg.ml−1 (87% in group 2 and 93% in group 3 vs. 38% in group 1, p < 0.01). Premedication was also associated with the requirement of a lower mean target concentration for induction, a lower induction dose and a shorter time to induction. There were no significant between-group differences in discomfort on infusion or target concentration during maintenance. For short ambulatory procedures, the recommended initial target concentration of propofol is 4 μg.ml−1 in premedicated and 6 μg.ml−1 in unpremedicated patients.