In a simple reaction time task, key-pressing responses to unpatterned light stimuli presented in the right and left visual fields were faster for the hand ipsilateral to the visual stimulus than for the hand contralateral to the stimulus. The superiority of the ipsilateral reactions was seen also when responses were made with the hands crossed, so that such a superiority cannot be attributed to spatial compatibility between the side of the stimulus and the position of the response device. Similar results were obtained in a reaction time task of the Donders's c-type. It is argued that these laterality effects in visuomotor reaction time tasks are best explained by the anatomical relationships between the visual fields, the cerebral hemispheres, and the hands. The possible interaction between these effects and the effects of spatial compatibility between side of stimulus and side of response device are discussed in relation to reaction time tasks of the Donder's b-type.