Melanin synthesis of follicular melanocytes is strictly coupled to the growth stage of the hair cycle (anagen), ceases during follicle regression (catagen), and is absent throughout the resting stage (telogen). Having previously characterized the expression and activity of melanogenesis-related proteins during the telogen-anagen transition of the murine hair cycle (JID 96:172, 1991), we here report a biophysical and biochemical analysis of follicular melanogenesis during the anagen-catagen-telogen transformation of the C57 BL-6 mouse hair cycle. Tyrosinase activity and concentration as well as dopachrome tautomerase activity were compared with melanin synthesis, as measured by electron paramagnetic resonance spectroscopy (EPR). The visible changes in skin color and the histologically appreciable switch-off of melanin formation during the anagen-catagen transformation were accompanied by a steep decline in 1) the melanin-associated EPR signal of full-thickness mouse skin, 2) tyrosinase and dopachrome tautomerase activities, and 3) the skin concentration of 80-85-kD melanogenesis related protein and 66-68-kD tyrosinase protein. Telogen skin displayed a minimum of the EPR amplitude as well as of tyrosinase and dopachrome tautomerase activity detected. By EPR, only eumelanin was identified during all hair cycle stages. The gradual switch-off of melanogenesis during anagen VI started with an unexpectedly early decline of the EPR melanin signal, followed by dopachrome tautomerase activity and the concentration of 80-85-kD melanogenesis related protein. The initiation of catagen was characterized by a significant and rapid decrease in activity and concentration of tyrosinase, and was accompanied by a second drop in dopachrome tautomerase activity. Together, these biochemical and biophysical parameters of follicular melanogenesis serve as novel and differential markers for the imminent termination of anagen and the development of catagen. They also show that the switch-off of melanogenesis during the anagen-catagen-telogen transition is a stochastic process commencing already in mid anagen VI.