A theoretical formulation was derived for the dependence of bulk solute permeability, P, defined as net flux divided by concentration gradient, delta c, across any membrane in which solute concentration is controlling for net flux, delta J. According to this formulation, delta J is stimulated by increments in trans concentration, c2, in the range c2/c1 equals 0.0--0.1. Net flux of urea across toad bladder down concentration gradients was shown to be stimulated threefold by small increments in trans urea concentration. The theory also predicts that, in the absence of concentration gradients, tracer permeability, P, defined as tracer flux divided by tracer concentration, will be independent of c provided that P equals P, but will diminish with increasing c if P/P less than 1.P/P was not significantly different from unity for urea, and both P and P were independent of c in the absence of concentration gradients. However, P/P was significantly less than unity (0.90 and 0.85) for thiourea and mannitol, respectively. In conformity with theory, P (and also P) of these two solutes, measured as c was increased by 3--4 orders of magnitude, diminished progressively. These effects are more consistent with this formulation than with transport via a saturable carrier.