There is controversy in the literature concerning the action of tetrahydropterines on the enzyme tyrosinase and on melanogenesis in general. In this study, we demonstrate that tetrahydropterines can inhibit melanogenesis in several ways: i) by non-enzymatic inhibition involving purely chemical reactions reducing o-dopaquinone to L-dopa, ii) by acting as substrates which compete with L-tyr and L-dopa, since they are substrates of tyrosinase; and iii) by irreversibly inhibiting the enzymatic forms met-tyrosinase and deoxy-tyrosinase in anaerobic conditions. Three tetrahydropterines have been kinetically characterised as tyrosinase substrates: 6-R-L-erythro-5,6,7,8-tetrahydrobiopterin, 6-methyl-5,6,7,8-tetrahydropterine and 6,7-(R,S)-dimethyl-5,6,7,8-tetrahydropterine. A kinetic reaction mechanism is proposed to explain the oxidation of these compounds by tyrosinase.