Information about the action of vasopressin on the structure of the rat kidney is relatively meagre. Byrom (1939) has described the histological lesions in the rat kidney induced by enormous amounts of Pitressin as 'direct infarction or ischaemic degenerative changes short of infarction of specialized parenchyma...', but emphasized that 'after smaller doses (5–20 units) no definite lesions are seen unless injections are repeated daily or twice daily for at least 3 days'. The lesions were attributed to vascular spasm. However, from experiments on the effect of small doses of Pitressin on the renal circulation of hamsters (Thurau, Deetjen & Kramer, 1960) and rats (Fourman & Kennedy, 1966) it has been suggested that renal blood flow is affected, possibly because of vascular constriction. The present study was designed to determine the dose of Pitressin which induces histological lesions of the rat kidney. Albino (Wistar) rats of both sexes, weighing 180–240 g,