Seed lots of soybean cultivars Biloxi, Lee-68, Santa Rosa, IAC-3, Davis and Hardee were exposed to 48°C for 0, 48, 96, 144, 240 and 288 hours to study the effects of this temperature upon germination and occurence of cotyledon necrosis. Soybean plants were grown under greenhouse conditions, the pods were hand harvested and the germination chamber was kept at 20-30°C. Seeds which were not submitted to 48°C presented seedlings without cotyledon necrosis symptons. On an average, there was a approximately linear reduction in total germination from 92 to 23% as the period of exposure was extended. On the other hand, increasing periods of seed exposition to 48oC caused a higher percentage of seedlings with necrosis. Biloxi showed the best germination and the lowest necrosis index when compared with the other cultivars. Confirming previous observations Hardee showed as the most heat sensible cultivar in relation to germination as well as to necrosis. A significant correlation (r = - 0.74) was observed between germination percent and seedlings with necrosis when the seeds were exposed to 48, 96, 144 and 192 hours to 48°C. There was a tendency of cultivars with higher protein content to present better germination.