Injury to ‘Prelude’ and ‘Manhattan II’ perennial ryegrass (Lolium perenne L.) was measured as percentage of electrolyte leakage from leaf segments after stress to determine the influence of prestress growth temperature and poststress temperature on heat tolerance. The temperature required to cause 50% cell solute efflux was 59.5°C for ‘Prelude’ and 56.5° for ‘Manhattan II’, when measured immediately after stress treatment. However, electrolyte leakage increased with time after termination of stress. When measured 24 hr after termination of stress, 52° caused 50% cell solute efflux from leaf segments of both cultivars. Injury levels 44 hr after 30 min at 50° were ≈ 12% and 89% when incubated at poststress temperatures of 7° and 35°, respectively. Incubation temperature following a 55° treatment did not affect electrolyte leakage rate in either cultivar. Greater injury occurred in both cultivars when grown at 25° than at 41°.