The rate of p-nitrophenyl palmitate (pNPP) hydrolysis in n-heptane by a lipase preparation from Pseudomonas cepacia was studied. In organic media, the reaction rate could be easily measured by spectrophotometry after extraction of the liberated p-nitrophenol by an alkaline water phase. Optimal conditions for substrate hydrolysis were selected and proposed for the easy and fast assay of lipase activity in organic media. Agitation and the amount of added water showed little effect on reaction rates. A temperature of 37 degrees C and substrate concentration of 50 mM were selected. The reaction rate in n-heptane was only 0.16% of that in water.