A fast X-ray diffraction technique has been used to study the osmotic reaction of frog rod outer segments to bleaching and to changes in the osmolarity and composition of the outer medium. Dissected excitable retina and isolated outer segments have been used. Upon bleaching in isotonic Ringer only a small transient diminution of the disc repeat distance is observed in isolated rods. Disorder and slow swelling reactions are also observed in the intact cells. In calcium-free Ringer the light-induced shrinkage is considerably enhanced. In the intact cell this is interpretable as due to the switching off of a large sodium dark current. The persistence of the effect in isolated outer segment suggests the existence of an active ionic efflux from this part of the cell. Upon hypotonic shock, bleached rods swell more than dark-adapted ones. The difference, however, appears only in a slow component of the osmotic kinetics, a few minutes after the shock. Upon hyertonic shock, part of the rods, even in the "intact" excitable retina, appear to be leaking. Those cells which are intact are impermeable to all the solutes added to increase the osmolarity: NaC1, KC1, Sucrose, Melezitose. No light dependence of the response to a hyperosmotic NaC1 shock is detectable. The discs are osmotically reactive, even when the outer cell membrane of the rods is leaking. Assuming the discs to be perfect osmometers, a thickness of 20 plus or minus 5 A is estimated for the liquid layer inside the discs.