The effect of cold ambient temperature on the energy cost and thermoregulation during exercise was studied in 18 male Sprague-Dawley rats accustomed to running for 2 weeks. For measuring T(b) and ensuring an unrestrained exercise performance, the rats were implanted with intraperitoneal telemetric radio transmitters. The rats ran at three submaximal exercise intensities (10, 15 and 20 m.min-1) at different T(a) values (22 degrees, 0 degree, -10 degrees and -20 degrees C) for 30 min. VO2 and Tb were continuously measured. During exercise at 0 degree C a 100% substitution of thermoregulatory heat production by exercise thermogenesis was observed. At T(a) values below 0 degree C the physical strain of exercise increased considerably: Vo2 increased 34-61% at -10 degrees C and 51% at -20 degrees C compared to the corresponding exercise intensity at T(a) 22 degrees C. A partial substitution of thermoregulatory heat production by exercise thermogenesis was observed in the cold. At -10 degrees C the cost for thermoregulation during exercise was 28% and at -20 degrees C 31% lower than at rest at the corresponding T(a) values. The rat could not maintain normal Tb during exercise below 0 degrees C. The T(b) dropped 1.5-1.8 degrees C at -10 degrees C and 2.5 degrees C at -20 degrees C. It is concluded that exercising below 0 degree C at submaximal levels (below 50% of VO2max) partially substitutes for thermoregulatory costs but is thermally unfavorable for the small-sized rat.