Urinary calculi composed of Ca oxalate were produced in the male albino rat by the feeding of NH4 oxalate (incidence about 10%) and of a combination of NH4 oxalate and NH4Cl (incidence about 69%). The basic diet used was nutritionally adequate for the rat and stone was absent in the control group. The feeding of NH4Cl alone was not associated with urinary stone formation. The tip of the papilla appeared to be the site of origin of stones attached to the renal papilla, with major growth of stone directed towards the base of the pyramid. The common presence of a facet in stones found attached to the renal papilla, loose in the renal pelvis, in the ureters, and in the bladder, suggests a common site of origin of the stones produced; the renal papilla. Papillary plaques were found to contain Ca oxalate and an additional substance thought to be protein.