The study was designed to elucidate the interrelationships between behavioral and autonomic thermoregulation in three common strains of the laboratory rat. In one experiment 8 adult rats of the Sprague-Dawley (SD), Long-Evans (LE), and Fischer (FCH) strains were repeatedly placed in a longitudinal temperature gradient while their preferred ambient temperature (Ta) was recorded. The mean preferred Ta's (plus or minus S.E.) for the SD, LE, and FCH strains were 24.9 plus or minus 0.4, 19.8 plus or minus 0.3, and 23.4 plus or minus 0.3 C, respectively. In another experiment individual adult rats of the same strains were placed in an environmental chamber thermostabilized at a Ta of 14, 16, 18 ... or 36 C for 90 min while metabolic rate (MR), evaporative water loss (EWL), thermal conductance (C), and colonic temperature (Tcol) were determined. All three strains exhibited a minimal MR at a Ta of 30 C. As Ta decreased below 30 C, MR increased in a non-linear fashion. EWL and C were minimal at cool Ta's and increased gradually with an elevation in Ta.