In the process of studying the inhibition of dopamine-beta-hydroxylase by cysteine, we observed an interesting relationship between cysteine and catalase. This suggests that two different patterns of the inhibition of dopamine-beta-hydroxylase exist; the one is reversible inhibition, the other is irreversible inhibition. In the present experiments, we have particularly investigated the irreversible inhibition of dopamine-beta-hydroxylase by cysteine, and the following conclusions were drawn: (a) The rate of the inhibitory effect of cysteine pretreatment on dopamine-beta-hydroxylase is independent of a function of the temperature, while at temperature above 20 degrees C the extent of the reversibility of cysteine inhibition by N-ethylmaleimide is temperature dependent. (b) The rate of inhibitory effect is dependent on the time of cysteine pretreatment at 37 degrees C, and the ability of N-ethylmaleimide to reverse the cystine-induced inhibition is gradually diminished by increasing the time of cysteine pretreatment. (c) The inhibitory effect of cysteine pretreatment (37 degrees C, 10 min) is not reversed by the addition of Cu++. (d) In the presence of higher concentrations of catalase, the cystein-induced inhibition is recovered after the addition of equimolar concentration of N-ethylmaleimide. (e) Both inhibitions of cysteine on dopamine-beta-hydroxylase with and without pretreatment were the noncompetitive type to the substrate, tyramine.