Eight men aged 60-65 years and six men aged 20-25 years, wearing only swimming trunks, were exposed to an air temperature of 17 degrees C and 45% R.H. in each of the four seasons. The increase in the rate of metabolic heat production (% delta M) for the older group in the cold test was significantly higher in summer and autumn than in winter and spring (P < 0.05), but did not differ in the young group between seasons. Compared to the young group the % delta M was significantly greater for the older group (due to a marked increase in four individuals) in summer and autumn (P < 0.04). At the end of the period of cold exposure, the decrements of rectal temperature (delta Tre), mean skin temperature (Tsk; due to a marked decrease in four individuals) and foot skin temperature (Tfoot) were significantly greater for the older group compared to the young group at all times of the year (P < 0.003). Seasonal variations in the two groups were similar, e.g., the delta Tre gradually became smaller from summer to winter (P < 0.05) and then increased slightly in the spring (P = 0.07). Tfoot for both groups decreased from summer to autumn (P < 0.01) and remained unchanged subsequently. No seasonal variations were observed for Tsk in either group. The increase in diastolic blood pressure (BPd) during the test was significantly smaller in winter in both groups (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)