The possibility of demonstrating the activity of respiratory enzymes in paraffin sections was studied. Unfixed pieces of nervous tissue were incubated at 4 degrees C, 20 degrees C, 37 degrees C and 56 degrees C for various periods ranging from 1 to 24 hours. After dehydration, the tissue pieces were mounted in paraffin. The paraffin sections obtained there of were then tested with respect to the range of penetration of the substrate into the incubated tissue samples (as judged from the resulting histoenzymic reaction), and for the distinctness with which the localization of the histochemic reaction could be assessed. From the results it may be concluded that it is possible, under well defined conditions, to demonstrate the activity of dehydrogenases in paraffin sections. The resulting morphological pictures permit a much better localization of the histoenzymic reaction products than those obtained from cryostat sections. Optimal results are obtained when tissue fragments, about 1 mm in diameter are incubated for 24 hours at 4 degrees C.