A fast light-induced light-scattering transient, previously found in rod outer segment suspension, the so-called P-signal (Hofmann, K.P., Uhl, R., Hoffmann, W. and Kreutz, W. (1976) Biophys. Struct. Mechanism 2, 61–77), is described in more detail. The effect has the same action spectrum as rhodopsin bleaching. It is not regenerated with 11-cis retinal. The response is not linear with light-intensity for flashes which bleach more than 2.0% of rhodopsin; it saturates at an intensity corresponding to 15% rhodopsin bleaching. The wavelength- and scattering angle dependence lead to the conclusion that the change in light-scattering reflects a shrinkage of an osmotic compartment of the rod outer segment. The only compartment which we found to be intact in our rod outer segment preparations was the disc or rod sac; therefore, the effect must be attributed to a light-induced shrinkage of the rhodopsin-containing disc organelles. The overall effect (15% of rhodopsin is bleached) is in the range of 0.5–1.5% of the original volume. A light-induced passive cation-efflux from the disc, e.g. of Ca 2+, can be ruled out as a possible molecular origin of the disc-shrinkage in our preparations.