The permeability of the toad urinary bladder to 22 nonelectrolytes was obtained from measurements of radioactive tracer fluxes. The permeability coefficients (P's), after suitable corrections for unstirred layers, were proportional to the olive oil/water partition coefficients for the majority of the molecules (Palpha Koill.3). In the absence of chain branching, inductive effects, and intramolecular hydrogen bonding effects, a hydroxyl group reduced P an average 500-fold and a methylene group increased P an average four fold. Branched chain solutes were less permeable than their straight chain isomers, and small solutes, polar and nonpolar, exhibited higher rates of permeation than expected from the relationship between P and Koil. (Over the molecular size range 18-175 cc/mole Palpha(Molecular Volume)-2.7.) The high rates of permeation of small molecules are consistent with diffusion through a highly organized lipid structure. Large polar solutes, e.g., sucrose, appear to pass across the epithelium via an extracellular shunt pathway. The apparent activation energies (Ealpha) for the permeation of 16 select molecules were obtained from permeability measurements over the temperature range 2-32 degrees C. Linear Arrhenius plots (i.e., log P/T-1) were obtained for all molecules after unstirred layer corrections. In the absence of these corrections "phase transitions" were seen for molecules with very high P's (P greater than 300 X 10(-7) cm/sec), but these are simply due to diffusion limited permeation. Ealpha increased by 2.5-3.6 kcals/mole with the introduction of each additional methylene group into a molecule, and decreased by up to 9 kcals/mole for the addition of a hydroxyl group. Qualitatively similar results were obtained in preliminary studies of olive oil/water partition coefficients. Arrhenius plots of the toad bladder conductance over the temperature range 2-32 degrees C yield apparent activation energies of 4-5 kcals/mole which is identical to that found previously for "leaky" epithelia.