In cultured chick embryo liver cells, cyproterone and cyproterone acetate, synthetic anti-androgenic steroids, were found to be potent inducers of mitochondrial delta-aminolevulinic acid (ALA) synthetase, the rate-limiting enzyme in the heme biosynthetic pathway. Both steroids produced a detectable increase in enzyme activity at 5 muM and a maximal stimulation, 36-fold for cyproterone and 29-fold for cyproterone acetate, at 55 muM. The dose-response curves of the steroids differed, however, in that cyproterone acetate produced a greater mean stimulation of the enzyme at concentrations less than approximately 25 muM, whereas, at higher concentrations, cyproterone was the more effective inducer. Increased activity of ALA synthetase was not apparent until about 12 hours after the addition of cyproterone, and maximal activity was not achieved until 20-24 hours. The induction of ALA synthetase by these anti-androgens was prevented by actinomycin D, cordycepin, anisomycin, cycloheximide, and puromycin. These results suggest that new RNA and protein synthesis are necessary for enzyme induction. The cyproterone-mediated induction of the enzyme was inhibited 50% by 2 muM heme, the putative physiological inhibitor of ALA synthetase. These antiandrogens, unlike other potent steroid inducers of this enzyme in chick embryo liver, do not possess either a 5beta-pregnane or 5beta-androstane nucleus. The stimulation of hepatic ALA synthetase represents the first example of a direct effect of these steroids on enzyme induction.