Glucose-6-phosphate dehydrogenase and lactate dehydrogenase isoenzymes in tumors, produced by administration of 7,12-dimethylbenz(a)anthracene to Sprague—Dawley rats, were studied at 0, 3 and 5 days after oophorectomy. Of the two major glucose-6-phosphate dehydrogenase bands, only one was responsive to oophorectomy and showed a striking decrease in activity as reflected by specific staining after polyacrylamide gel electrophoresis. Likewise, the decrease noted in lactate dehydrogenase activity was found to be most pronounced in the muscle types during this 5-day period. The results indicate that some isoenzyme species are under hormonal control in these neoplasms and change prior to any significant regression of tumor mass.