Connecting tubules were dissected from rabbit kidneys and perfused in vitro with a solution identical to that in the bath. Transepithelial sodium fluxes were measured with 22Na. The mean lumen-to-bath flux was 33.7 +/- 3.4 peq . cm-1 . s-1 and the mean bath-to-lumen flux was 13.4 +/- 3.4 peq . cm-1 . s-1. Thus, the net sodium transport was 20.3 peq . cm-1 . s-1, which is approximately 3--4 times higher than the one previously measured in cortical collecting ducts under similar conditions. The sodium permeability calculated from the bath-to-lumen measurements corrected for mean voltage was 1.22 x 10(-5) cm .s-1. The mean transepithelial electrical resistance was 31 omega . cm2. This sodium permeability is much higher and the electrical resistance much lower than those in cortical collecting ducts. Spontaneous transepithelial voltage was oriented negative in the lumen and ranged up to -42 mV at low perfusion pressures. When the perfusion pressure was increased, the voltage decreased to approximately -5 mV without any measurable change in the electrical resistance or the lumen-to-bath sodium flux.