We have determined the permeability properties of freshly isolated frog rod outer segments by observing their osmotic behavior in a simple continuous flow apparatus. Outer segments obtained by gently shaking a retina are sensitive but nonideal osmometers; a small restoring force prevents them from shrinking or swelling quite as much as expected for ideal behavior. We find that Na(+), Cl(-), No(3) (-), glycerol, acetate, and ammonium rapidly enter the outer segment, but K(+), SO(4) (=), and melezitose appear impermeable. The Na flux is rectified; for concentration gradients in the physiological range, 2 x 10(9) Na(+) ions/sec enter the outer segment, but we detect no efflux of Na(+), under our conditions, when the gradient is reversed. Illumination of the outer segment produces a specific increase in the resistance to Na(+) influx, but has no effect on the flux of other solutes. This light-dependent Na(+) resistance increases linearly with the number of rhodopsin molecules bleached. We find that excitation of a single rhodopsin molecule produces a transient ( approximately 1 sec) "photoresistance" which reduces the Na(+) influx by about 1%, thus preventing the entry of about 10(7) Na(+) ions. At considerably higher light levels, a stable afterimage resistance appears which reduces the Na influx by one-half when 10(6) rhodopsin molecules are bleached per rod. We have incorporated these findings into a model for the electrophysiological characteristics of the receptor.