The Doppler shift frequency spectrum (DSFS) of laser light scattered from red blood cells (RBC) moving in the microcirculation of the optic nerve head was recorded in normal volunteers by means of a fundus camera laser Doppler velocimeter. The width of the DSFS, which varies in proportion to the speed of the RBC, was characterized by the parameter .alpha.. With the use of a model for the scattering of light by tissue and RBC and for the RBC velocity distribution, values of .alpha. recorded at normal intraocular pressure (IOP) suggest that the RBC that contribute to the Doppler signal are flowing in capillaries. The parameter .alpha. varied markedly with the IOP and with the phase of the ocular pressure pulse at elevated IOP. The return of the speed of RBC toward normal, which is observed after a step increase of IOP above normal and after a step decrease below normal, was attributed to an autoregulatory response of the optic nerve circulation.