Techniques based on a copper ferricyanide method were developed for the cytochemical localization at the ultra structural level of NAD- and NADP-linked glutamate dehydrogenases (NAD-GDH, NADP-GDH), the latter enzyme being specifically involved in mushroom cap development. These techniques were validated using malate, lactate and succinate dehydrogenases (MDH, LDH, SDH). Tests were applied to unfixed and formaldehyde-fixed material from three developmental stages of fruit bodies of Coprinus cinereus (Schaeff.: Fr.) S.F. Gray sensu Konr. (1) before spore formation (stage 2), (2) with young spores (stage 3), and (3) with mature spores (stage 4). Spectrophotometric assay showed that NADP-GDH was still detectable after 15 min formaldehyde fixation and approximately 9 and 80% activity of NAD-GDH and MDH, respectively, was preserved. At all stages of development MDH, LDH and SDH activities were localized in mitochondria or mitochondrion-like bodies. In contrast, both NAD-GDH and, especially NADP-GDH, were localized in cytoplasmic vesicles, mainly in basidia and very young spores, sometimes being found adjacent to the cell walls, suggesting that these enzymes are transported to the vicinity of the plasma membrane. Such a location would be consistent with enzymological data, indicating that NADP-GDH contributes to an ammonium scavenging system. Ultra structural localization of this system at the peripheral region of basidia suggests it may be linked to the synthesis and/or assembly of cell wall components and may protect the meiotic apparatus against ammonium inhibition.