The sites of lead phosphate precipitation in mouse bladder smooth muscle incubated with adenosine triphosphate and lead nitrate were studied by electron microscopy. The media constituents and incubating conditions were independently varied so that we could determine optimal conditions for histochemical demonstration of ATPase activity in agranular endoplasmic reticulum. Specimens of glutaraldehyde-fixed bladder muscle, frozen, cut into 10-40-micro sections, and incubated for 1 hr at 25 degrees C in medium containing 0.025 M ATP, 0.0025 M lead nitrate, 0.05 M magnesium chloride, and 0.09 M sodium acetate buffer at pH 6.2, exhibited microcrystalline deposits in agranular endoplasmic reticulum and pinocytotic vesicles. Lead salt deposition was also noted in terminal cisternae of sarcoplasmic reticulum in skeletal muscle similarly treated, suggesting that the organelle systems in the two types of muscle cells subserve a common function.