It has been believed that post-absorptive resting metabolic rate (RMR) is invariable provided room temperature is maintained not far from 20 degrees C. Some recent works pose a problem of variability of post-absorptive RMR under conditions of room temperature within the comfortable thermoneutral range; a thermogenic response to mild cold (around 20 degrees C) increases RMR. For a total of 23 male subjects between the ages of 22 and 29 years, determination of post-absorptive and post-prandial RMRs were made after the same subjects wearing standardized clothing stayed overnight on four separate occasions in a room controlled at 20 and 25 degrees C in both winter and summer seasons. RMRs at 20 degrees C in winter were significantly greater by 6-9 per cent than those at 25 degrees C in the same season, and those at 20 degrees C and 25 degrees C in summer. These differences were commonly observed in the post-absorptive RMRs and post-prandial RMRs. No statistical differences were detected among the RMRs on the other three occasions (25 degrees C in winter, 20 degrees C and 25 degrees C in summer). According to a three-way ANOVA, the effects of room temperature, season (outdoor temperature) and time course on RMRs were statistically significant. The effects of room temperature and of season on RMRs were independent of time course, and interacted with each other. These results suggest that 20 degrees C room temperature is not sufficient to determine definite RMR. At the lower room temperature, outdoor temperature is a possible factor that can affect RMRs.(ABSTRACT TRUNCATED AT 250 WORDS)