The discovery that glomeruli are normal in size and tubules 30 percent shorter than normal in kidneys of the hypothyroid (I131) rat will be exploited as a new approach to study of the physiology of glomerulotubular balance and intrarenal heterogeneities. Evidence will be sought for dysfunctions, other than the characteristic hypernatriuresis, attributable to glomerulotubular imbalance. Measurements will be made of tubular reabsorption of glucose over a range of filtered loads during sodium deprivation and excess, with determinations of SNGFR, tubular filtrate velocities (Hanssen), overall inulin and PAH clearances, and nephron dimensions. Changes in these parameters will be delineated in hemorrhagic hypotension, urinary obstruction, and renal hypertension (renal artery constriction with and without contralateral nephrectomy) in hypothyroid rats, together with special attention to patterns of nephron functional redistributions. Plasma renin, angiotensin II, and renin substrate will be followed to assess the possible effect of relative hyper-filtration upon macula densa function. The role of growth in pathogenesis of the short nephron disorder will be examined by the same methods in hypothyroid puppies and compensatory renal hypertrophy in hypothyroid rats. A search will be made for evidence of the disorder in adult dogs and patients with hypothyroidism.