A simple fluorometric assay method as well as a sensitive HPLC method for determination of bunitrolol, a beta-adrenoreceptor blocking drug, and its 4-hydroxylated metabolite is described. More than 90% of bunitrolol metabolized was accounted for by the formation of 4-hydroxybunitrolol in rat hepatic microsomes. Bunitrolol 4-hydroxylation required NADPH, and was inhibited by CO, proadifen and metyrapone, indicating that this reaction is mediated by cytochrome P450. This reaction was also inhibited by cimetidine competitively, and the inhibition constant, Ki, was 40 +/- 11.5 microM (mean +/- s.e. n = 3).