Unidirectional calcium flux (JCa) in the superficial pars recta and thin descending limb of Henle (DLH) was examined by the isolated tubule microperfusion technic using 45Ca as the isotopic tracer. In the pars recta sequential measurements of lumen-to-bath flux (JlbCa) and bath-to-lumen flux (JblCa) revealed: JlbCa 22.4 +/- 4.18, JblCa 7.97 +/- 1.95, and calculated net efflux of calcium (JnetCa 13.0 +/- 1.74 peq min-1 mm-u. To measure JnetCa directly, 45Ca of identical specific activity was used to bathe and perfuse the tubule. These studies revealed: JlbCa 14.1 +/- 1.33, JnetCa 11.2 +/- 1.15, and calculated JblCa 2.91 +/- 0.49 peq min-1 mm-1. The addition of ouabain (10 microM) resulted in a rise in potential difference and a fall in water absorption, but not a statistically significant change in JnetCa. Tubules studies at 25 degrees C bath temperature, showed no significant JnetCa, and upon heating the bath to 37 degrees C, showed JnetCa of 3.75--5.00 peg min-1 mm-1. Unidirectional and net efflux studies in six DLH showed no significant transport of calcium. These studies demonstrate substantial active absorption of calcium by the superficial pars recta, which is not inhibitable by ouabain but is inhibited by lowering bath temperature to 25 degrees C. No significant calcium transport was found in the DLH using identical technics.