Experiments were conducted in late summer and winter with 80 young and elderly Danish subjects exposed for 3.5 hours in a climate chamber to the temperature increasing from 24°C to 35.2°C at a rate of 3.7K/h. Psychological and physiological measurements were performed during exposure and subjects assessed comfort and acute health symptoms. Thermal sensation increased with increasing chamber temperature and did not differ during late summer and winter exposures. Skin temperature increased with increasing temperature and was slightly but significantly higher in the late summer in the first half of exposure. Core temperature started to increase, when the chamber temperature reached about 28oC, earlier in winter than in the late summer. Thermal environment was assessed to be slightly less acceptable in winter only until chamber temperature reached about 28oC; acceptability systematically decreased with increasing temperature. Difficulty to concentrate increased with increased temperature and the self-estimated ability to perform work decreased; subjects reported being sleepier. Severity of headache and difficulty to concentrate was in winter slightly but systematically higher, subjects reporting also to be sleepier. Heart rate slightly increased during exposure, and SpO2 and ETCO2 began to decrease while core temperature started to increase. Performance of Tsai-partington test and addition test improved during exposures due to learning though lesser in winter. Results show negative effects of the temperature ramp, being somewhat higher in winter than in the late summer.