The inhibitory effects of bulbocapnine, an aporphine isoquinoline alkaloid, on bovine adrenal tyrosine hydroxylase (TH) were investigated. Bulbocapnine had an inhibitory effect on the enzyme (43.6% inhibition at concentration of 200 microM). Bulbocapnine exhibited uncompetitive inhibition on bovine adrenal TH with a substrate L-tyrosine. The Ki value was found to be 0.20 mM.