We describe a novel technique for the histochemical and cytochemical demonstration of glucose-6-phosphatase activity. In this method, lead is replaced by cobalt. After activity of glucose-6-phosphatase, cobalt phosphate Co3(PO4)2 is formed, and in the presence of ammonium sulfide (NH4)2S, the precipitate is transformed into a sulfide that fixes osmium and provides good electron density. Glucose-6-phosphatase activity was determined mostly in rat kidney cells, but controls were also performed in liver cells. A strong reaction was seen in proximal tubule cells, but the reaction was weak in distal convoluted tubule cells. This technique showed the same endoplasmic reticulum (ER) organization in proximal and distal nephron as that seen with the osmium impregnation technique. In collecting tubules, intercalated cells had irregular reactivity, while principal cells had none. Our results indicate that the cobalt technique is valid, reliable, and sensitive enough to detect low glucose-6-phosphatase activity. Moreover, the technique can be used with 1-mm-thick specimens and obviates the need for use of frozen tissue sections.