Avian kidneys show features of both mammalian and non-mammalian kidneys, possessing cortical reptilian-type (RT) and medullary mammalian-type (MT) nephrons, blood supplies from renal arterial and renal portal systems, and a primitive macula densa. To determine how these morphological characteristics contribute to unique renal functions, we examined first a possible functional link between the renal tubules and the preglomerular vasculature and, second, the function of the loop of Henle of the MT nephrons. Infusion of 5% NaCl (2.0 ml/kg/h) into the renal portal system of the pullet Gallus domesticus caused diuresis and natriuresis in the infused side, whereas infusion of the same dose into the systemic circulation showed no effect. Infusion of 10% NaCl at a higher flow rate into either the renal portal or systemic routes caused hypernaturemia and a prolonged antidiuresis, presumably due to a release of arginine vasotocin. Plasma renin activity (PRA) decreased during renal portal infusion of hypertonic saline, suggesting that increases in intratubular or peritubular NaCl levels may suppress PRA. The thick limb of the loop of Henle (TLH) isolated from the MT nephron of the quail Coturnix coturnix revealed a lumen-positive, furosemide-sensitive transepithelial voltage (Vt) that requires the presence of both Na and Cl. Net water flux (Jv) was nearly zero when the TLH was perfused and bathed with isosmotic solution. Net water flux increased only slightly and osmotic water permeability (Lp) was low when the osmotic gradient was imposed. Vasotocin altered neither Vt, Jv, nor Lp. Chloride efflux was higher than Cl influx, and net Cl absorption was comparable to that of the TLH of the mammalian kidney.(ABSTRACT TRUNCATED AT 250 WORDS)