Mechanisms underlying direction selectivity were studied in V1 of alert fixating macaque monkeys. Some direction-selective cells showed delayed asymmetric inhibition, some showed a shifting excitatory time course across the receptive field, and some showed both. Both the direction of the spatial offset of the inhibition and the direction of the shift in excitatory response time course correlated with the cells' preferred directionality. The delayed asymmetric inhibition may contribute to the shifting response time course. The data suggest that asymmetric inhibition is the major determinant for directionality in these cells, though both mechanisms could contribute. Based on this physiology, a simple, single-cell model is proposed, consistent with the known anatomy of some direction-selective cells.