The Measurement of Heat Loss From the Rat's Tail

A simple method is described for measuring variation in the rate of heat loss from the tail of unanæsthetized rats with Hatfield‐Turner heat flow discs. The method was tested by giving l‐isoprenaline to adult rats acclimated to 20 or 3° C. The thermogenic response to this drug at an ambient temperature of 20° C. was increased four‐fold by cold‐acclimation. At 20° C. both the ‘resting’ and maximum rates of heat loss from the tail were greater in the 3° C. acclimated rats. The rate of heat loss from the tail increased abruptly to its maximum when the colon temperature reached a critical level of about 39·2° C. This response to l‐isoprenaline was not part of a general vaso‐dilatation diie to the drug.