The interaction between roxithromycin and midazolam was investigated in a double-blind, randomised crossover study of two phases. Ten healthy volunteers were given roxithromycin (300 mg) or placebo once daily for 6 days. On the sixth day they ingested 15 mg midazolam. Plasma samples were collected and psychomotor performance measured for 17 h. Roxithromycin administration significantly increased the area under the plasma midazolam concentration-time curve from 8.3 to 12.2 micrograms.ml-1.min and the elimination half-lives from 1.7 to 2.2 h. In psychomotor performance only minor differences were seen between the treatments in one of the measured psychomotor parameters. Thus, in contrast to the strong interaction between erythromycin and midazolam, the interaction between roxithromycin and midazolam appears less likely to be clinically significant.