Receptive field sizes of motion detector units in the human visual system were determined using a summation technique. Contrast sensitivity was measured for detecting the direction of motion of a drifting (8 Hz) sinewave grating (0.01-30.0 c/deg) multiplied by a stationary Gaussian envelope, for various widths of the Gaussian envelope. For each test spatial frequency, sensitivity increased linearly with aperture width up to a certain limit, and thereafter at a rate consistent with a model incorporating probability summation over space and between channels. The limit of linear summation designates the limit of the receptive field. Results show that the receptive field size varies with spatial frequency, from 2' arc at high spatial frequencies to as large as 7 deg at low frequencies. The change in field size was progressive. The smallest aperture width (delta W) for directional discrimination was also measured. Results show delta W to vary from 0.03 cycles at low spatial frequencies to 0.30 cycles at high frequencies.