Cyclic fluctuations were studied in the activity of oxidoreductases playing a role in the major energy metabolic pathways, lysosomal and non-lysosomal hydrolases and some non-enzymatic cytochemical components demonstrable in different developmental physiological or pathophysiological phases of human endometrium. The total scope of the study involved 170 tissues and cytological specimens. The cytological material included microcurettings, aspirates, brush preparations and tissue prints. An evaluation of the usefulness of the application of enzyme cytochemistry to cytological material is included. The most important results were a cyclic fluctuation and a progestagenic controlled increase in the activity of many oxidoreductases, especially the NADPH regenerating enzymes of the pentose phosphate pathway, and of the NADP+ dependent isocitrate dehydrogenase. The histochemical evaluation of the activity of these NADP+ linked enzymes can therefore be recommended for the evaluation of the physiological status of the endometrial cells, especially in patients with infertility problems.