The activities of three enzymes of the hexose monophosphate pathway (HMP) were measured in frozen samples of rat cerebral cortex and ethylnitrosourea-induced tumors of the rat nervous system. Results show that in most tumors, adequate amounts of glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and transketolase (TK) were present so as not to be rate-limiting for utilization of the HMP. When enzyme activites were expressed on the basis of fresh weight, TK was increased in most tumors as compared with cerebral cortex; G6PDH and 6PGDH were more variable, showing either higher or lower activities in tumors as compared to control. Tumors in general had a lower protein content than cerebral cortex. When activities were expressed in terms of this protein content, mean values for the dehydrogenases in all groups of tumors were higher than the average value in cortex, indicating that G6PDH and 6PGDH were spared in tumors relative to other proteins. TK activities in tumors, expressed in terms of cytosol protein were lower than or in the same range as cortex. Increased activities of HMP enzymes in some tumors indicated that the potential activity of the HMP in some (but not all) tumors of the nervous system is greater than that of cerebral cortex.