1. Ca2+ transport was examined in segments of rabbit pars recta isolated from the renal cortex by the technique of in vitro perfusion. 2. The bidirectional Ca2+ flux was determined by adding 45Ca to the bath and to the perfusate sequentially. Five tubules were perfused with Ringer-HCO-3 solution containing 0.5% albumin and bathed in the same solution, which maintained the ionized Ca2+ at the same concentration in the perfusate and bath. The outflux of Ca2+ was 169 +/- 18 pEq cm-1 min-1, while the influx was 53 +/- 13 pEq cm-1 min-1. The difference between these two numbers, the net Ca2+ absorption, is equal to 116 +/- 19 pEq cm-1 min-1. The flux ratio, 3.79 +/- 0.87, was significantly higher than 0.91 +/- 0.01 predicted by the Ussing equation from the transtubular potential difference (PD = -1.3 +/- 0.2 mV). 3. The effect of ouabain and transtubular PD on Ca2+ outflux was studied with rabbit serum ultrafiltrate as perfusate and rabbit serum as the bath. When 10(-5) M ouabain was added to the bath the transtubular PD fell from -2.4 +/- 0.3 mV to -0.65 +/- 0.06mV and fluid absorption decreased from 0.83 +/- 0.15 to 0.42 +/- 0.2 nl min-1 mm-1, whereas Ca2+ outflux remained unchanged (before = 160 +/- 13 pEq cm-1 min-1, after = 167 +/- 7 pq cm-1 min-1). 4. When tubules were perfused with and Na+ -free solution (NaCl replaced by choline chloride), Ca2+ outflux, 170 +/- 170 11 pEq cm-1 min-1, was the same as that obtained with the normal perfusion solution. 5. These results indicate that: a) the pars recta has the capacity for net absorption of significant amounts of Ca2+ that is apparently independent of Na+ efflux; b)Ca2+ transport across the pars recta occurs by a mechanism other than simple diffusion and c), unlike Na+ transport, Ca2+ outflux is not sensitive to ouabain.