Colloidal dispersions of polycyclic aromatic hydrocarbons were found to inhibit glyceraldehyde-3-phosphate dehydrogenase (GPDH) and lactic acid dehydrogenase (LDH) after a short exposure of the incubation medium to laboratory lighting. The activation effect was observed with substituted 7-methyl derivatives of 12-methylbenz(a)anthracene also. One minute exposure of 7-methoxy methyl-12-methylbenz (a)anthracene (MeO-DMBA) to light was sufficient to produce a measurable amount of GPDH inhibition. The rate of enzyme inhibition was related to the pH of the dispersion. A small but significant effect of light on the inhibition of the enzymes by K-region epoxides was seen. Protein binding of the epoxides was not well correlated with the inhibition. The light effect was oxygen-dependent and was related to the hydrocarbon concentration. The active form decayed over a period of more than 25 days. The necessity for protecting reactions involving hydrocarbons and their derivatives from light is emphasized. © 1975.