Hair follicular tyrosinase activity was measured during hair growth in neonatal, pubertal, and adult C3H-HeAvy mice that show differences in coat color as a result of changes in the synthesis of eumelanin and pheomelanin. Tyrosinase activity increased during hair growth in all mice but higher levels were found at puberty, when the mice grow a dark, eumelanin coat of hair, than during early and adult life, when the hair follicular melanocytes produce mainly pheomelanin. This suggests that tyrosinase is more important for the synthesis of eumelanin than that of pheomelanin. The increased tyrosinase activity associated with eumelanogenesis in the pubertal mice could not be explained by enhanced transcription of the tyrosinase gene or enzyme synthesis and appeared to be the result of a post-translational activation. Such an activation of tyrosinase was lacking in the neonatal and adult mice; in the latter this was accompanied by a reduction in the glycosylation of tyrosinase and the proportion of enzyme associated with the melanosomal fraction. Our findings suggest that post-translational mechanisms are important control points in the regulation of tyrosinase and that differences in their level of activation are responsible for determining the patterns of melanogenesis in the C3H-HeAvy mice, but it is still not clear how these mechanisms are regulated. Although cyclic AMP increased tyrosinase synthesis it had no post-translation activating effect. The neonatal mice, unlike their pubertal and adult counterparts, also lacked dopachrome converting activity and TRP tyrosinase-related protein-1 expression.(ABSTRACT TRUNCATED AT 250 WORDS)