Unilateral post-ischaemic acute renal failure (ARF) was produced in rats by occluding the left renal artery and vein for one hour. Left renal function was assessed 1 or 2 h after the end of the period of ischaemia and the kidneys fixed by arterial perfusion. ARF was characterized by increased urine flow (8.1 ± 1.2 SEM #/min/100 g body weight, n = 14; controls 1.0 ± 0.1, n = 11), decreased urinary osmolality (335 ± 12 m Osm/kg, n = 14; controls 1885 ± 97, n = 11), and markedly reduced 3H inulin urine/plasma ratio (3.98 ± 0.56, n = 14; controls 499 ± 60, n = 9) and 3H inulin clearance (32.9 ± 7.0 μml;l/min/100 g body weight, n = 14; controls 505 ± 45, n = 9). Renal cortical blood flow, determined by the hydrogen desaturation technique, was less in animals with ARF (4.2 ± 0.4 ml/min/ml of cortex, n = 8) than in controls (5.4 ± 0.6, n = 5), but not significantly so. In vivo stereomicroscopic examination of the left renal surface in ARF revealed dilated proximal convoluted tubules and delayed passage of intravenously injected dye (lissamine green) through these tubules. Histological examination also showed dilated proximal convoluted tubules and cellular debris impacted in the terminal straight portions of proximal tubules and thin limbs of the loops of Henle. Light microscopic morphometric studies demonstrated significant dilatation of proximal convoluted tubules and Bowman's spaces, and significant narrowing of the lumina of distal convoluted tubules and cortical collecting ducts. The data support the pathogenetic significance of tubular obstruction in a polyuric model of ischaemic ARF in the absence of a marked decrease in renal cortical blood flow, and demonstrate the value of morphometric studies in experimental ARF. The usefulness of the hydrogen desaturation technique for the determination of renal cortical blood flow, reported herein for the first time in ischaemic ARF, is emphasized.