The Maillard browning reaction between carbohydrates and amines is part of an extensive series of reactions that is the basis for the brown color caused by the "sunless tanning" agent dihydroxyacetone in self-tanning products. The initial stages of the reaction are quite complex, but the ultimate products are brown polymers known collectively as melanoidins. We have now used electron spin resonance to show that radicals are produced in vivo by the Maillard reaction, initiated by treating the skin of hairless mice with a solution of dihydroxyacetone in buffer. Dihydroxyacetone was used as the carbohydrate because it is simple but highly reactive and is the only USFDA approved color additive for the production of a sunless tanning response on skin. Treated skin turned brown within 24 h and showed an electron spin resonance signal after sacrifice of the animal. The control sample, consisting of untreated skin from the same animal, remained its original pink color and had no electron spin resonance signal. In corresponding ex vivo experiments in which mouse skin was soaked in dihydroxyacetone solutions, it was conclusively demonstrated that the presence of the dihydroxyacetone was required for radical formation in skin. In both the in vivo and ex vivo reactions the electron spin resonance signal consists of a broad single line with a peak-to-peak linewidth of 15 Gauss and a g value of 2.0035. We suggest that dihydroxyacetone interacts on skin through a free radical mediated reaction similar to its in vitro reactions with amines and amino acids.