A man who had undergone forebrain commissurotomy (L.B.) and a man with agenesis of the corpus callosum (R.B.) judged whether pairs of spatially separated lights were successive or simultaneous. Stimulus onset asynchronies (SOAs) were 0, 17, 33, 50, 67, 83, 117, and 150 ms. When the lights were in opposite visual fields, the SOA at which the discrimination first reached a level significantly above chance was 150 ms for L.B., 67 ms for R.B., and 33 ms for "normal" participants. Results parallel evidence from reaction time studies in which estimates of interhemispheric transfer time for callosal agenesis patients lie between those of normal controls and those with surgical section of the forebrain commissures. L.B. also showed a left-visual-field deficit in the discrimination, though it was less marked than his deficit with bilateral presentations.