Effect of reduced acclimation temperature on responses of frog cold receptors^* * Present address: Department of Anatomy, Albert Einstein College of Medicine, Bronx, New York 10461, U.S.A.

1. 1. Acclimation of Rana pipiens to 9·5°C increased maximum dynamic sensitivity (impulses/ °C per sec), increased the range of static sensitivity and decreased the temperature of peak static sensitivity of cold receptors when compared to those of warm-acclimated animals. 2. 2. Cold receptors of cold-acclimated animals were markedly less sensitive to the application of epinephrine to the inner skin surface than were animals acclimated to 23°C, and showed much less enhancement of maximum dynamic sensitivity by sympathetic stimulation. 3. 3. Since circulating catecholamines are known to be elevated in the cold-acclimated amphibian, the present results may be interpreted as evidencing hormonal modulation of sensory input.