The reaction rate of glucose-6-phosphate dehydrogenase activity in liver sections from fed and starved rats has been monitored by the continuous measurement at 37 C of the reaction product as it is formed using scanning and integrating microdensitometry. Control media lacked either substrate or both substrate and coenzyme. All reactions were nonlinear; however, subtraction of either of the controls from the test response produced linearity. Differing responses in sections of livers from fed and fasted rats indicate that the appropriate control medium for use in the assay of this dehydrogenase is one lacking both substrate and coenzyme rather than a medium containing coenzyme. The reaction rate was the same with each of the final acceptors. Problems with the diffusion of the formazan of BPST and with the failure to precipitate the formazan of Neotetrazolium make Tetranitro BT and Nitro BT the tetrazolium salts of choice in quantitative dehydrogenase assays.