In 26 Wistar rats with surface glomeruli, the determinants of glomerular ultrafiltration and peritubular capillary uptake of proximal reabsorbate were studied before and during intravenous infusions of norepinephrine or angiotensin II. Regardless of whether renal perfusion pressure (AP) was permitted to increase, both hormones produced elevations in single nephron filtration fraction due to declines in glomerular plasma flow with little change in nephron glomerular filtration rate. The resulting large increases in the efferent arteriolar oncotic pressure, piE, were accompanied by equivalent increases in the mean glomerular transcapillary hydraulic pressure difference, deltaP. Equality of piE and deltaP, denoting filtration pressure equilibrium, obtained before and during infusion of either hormone. Por both hormones, when elevations in AP were allowed, marked and roughly proportional increases in the resistance to blood flow through single afferent and efferent arterioles occurred, whereas when increases in AP were prevented by partial aortic constriction increases in resistance were confined primarily to the efferent arteriole. Tespite the marked increases in piE, absolute rates of proximal tubule fluid reabsorption, on the average, were unchanged by these hormones due to the opposing effects of marked decreases in efferent arteriolar plasma flow rate and, to a lesser extent, increases in peritubular capillary hydraulic pressure.