The reduction in rice productivity as a result of elevated daily temperature due to climate change is a major concern for Thailand. This study aimed to investigate hydrogen peroxide and malondialdehyde (MDA) content and electrolyte leakage of rice seedlings grown from seeds exposed to different temperature (25°C, 35°C and 40°C) treatments over a short (one week) and long (two weeks) period before germination. Three rice cultivars were investigated, Dular, KDML105 and Riceberry. The experiment was designed in RCBD with six replications. The results indicated that Riceberry seeds produced a greater percentage of normal seedlings after both short- and long-term heat treatment (40°C). By contrast, KDML105 seeds exposed to 40°C for one and two weeks gave rise to the highest percentage of abnormal seedlings. The highest oxidative stress indicated by the accumulation of hydrogen peroxide was found in abnormal seedlings of cvs. KDML105 and Riceberry after short- and long-term heat (40°C) exposure, respectively. The effect of heat stress on membrane stability was indicated by MDA content and electrolyte leakage. MDA content was the highest in abnormal seedlings of cv. Riceberry after heat exposure for two weeks. High electrolyte leakage due to both short- and long-term high temperature treatment was found in abnormal seedlings of all the rice cultivars. Heat exposure to rice seeds at 40°C for one week induced the highest percentage of abnormal seedlings in KDML 105 coinciding with the highest hydrogen peroxide content and membrane damage. These results provide crucial information for consideration in breeding programmes for heat-tolerant rice cultivars.