Sodium currents were recorded from frog skeletal muscle by using fire-polished micropipettes to electrically isolate and voltage clamp a small patch of sarcolemma. Sodium current amplitude served as an assay for the number of functional sodium channels in the patch. With the pipette as a light guide, these channels were irradiated with ultraviolet (UV) light directed through a quartz fiber into the back end of the pipette. The UV light emerging from the pipette tip caused localized destruction of the sodium channels in the patch, reducing sodium current 3- to 5-fold during a 30-90 s irradiation. If sodium channels could diffuse laterally in the membrane, current from the patch should recover with time as fresh channels enter from neighboring areas. No such recovery was observed during observation for 1 hr after irradiation. Our results set an upper limit of 10(-12) cm2/s for the diffusion coefficient--1/1000th that of rhodopsin, a membrane protein in the cell membrane of retinal rods. It is suggested that sodium channels are anchored in the sarcolemma.