1. The ability to hydrolyse various phosphodiesterase substrates was examined in subcellular fractions of rat kidney and in serial slices of the kidneys of mouse, rat, guinea pig and ox cut from the cortex perimeter inwards. 2. d-Inositol 1:2-cyclic phosphate 2-phosphohydrolase could be clearly distinguished from phosphodiesterases which hydrolyse 2':3'- and 3':5'-cyclic AMP and p-nitrophenyl thymidine 5'-phosphate (phosphodiesterase I). The hydrolysis of sn-glycero-3-phosphorylcholine showed a distribution identical with that of particle-bound d-inositol 1:2-cyclic phosphate 2-phosphodiesterase, but there was a 30-fold difference in the ratio of enzyme activities between the rat and guinea pig. 3. In rat and mouse kidney, d-inositol 1:2-cyclic phosphate 2-phosphohydrolase is virtually all membrane bound and in the outer cortex, whereas in guinea-pig kidney the enzyme is almost entirely soluble and located throughout the kidney tissue. Some properties of the soluble enzyme are described. 4. Distribution and histochemical studies indicated that in the rat and mouse, phosphodiesterase I is associated with the brush borders of the straight portion (pars recta) of the proximal tubule, whereas inositol 1:2-cyclic phosphate 2-phosphohydrolase and probably glycerylphosphorylcholine diesterase are associated with the brush borders of the convoluted part of the tubule (pars convoluta).