The orientation selectivity of grating and line detectors is determined by subthreshold summation; that is, the reduction in contrast threshold of a grating or line test stimulus due to the presence of a subthreshold background grating is measured. For a 0.6 min line and also 5 c/deg and 10 c/deg test gratings the effect of the background grating is reduced to one half maximum by a tilt of about 3 deg; this orientation selectivity is more precise than that derived by other psychophysical methods. It is suggested that lateral inhibition between detectors contributes to the orientation selectivity derived by other methods but not by the present subthreshold method. The two-dimensional spatial extents of grating and line detectors are estimated. © 1973.