Visual acuity may be measured during the voluntary ocular pursuit of moving test objects. This visual function has been referred to as dynamic visual acuity. The apparent movement of the test object is produced by rotating a mirror in the desired plane of pursuit by means of a wheel and disk type variable speed drive. The range of angular velocities utilized is 10°/ to 170°/sec at the nodal point of the tested eye. It is shown that visual acuity deteriorates markedly and significantly as the angular velocity of the test object is increased. It is shown further that the relationship between visual acuity and the angular velocity of the test object may be described satisfactorily by the semiempirical equation Y=a+bx^3. It also is pointed out that individuals possessing similar static acuity may differ significantly in their dynamic acuity. The possible causes for the observed deterioration of acuity are discussed and it is concluded that imperfect pursuit movements of the eye result in a continued motion of the image on the retina. This motion results in reduced intensity contrast, which is a factor in producing loss in acuity.