Afterdrop, the continued fall of deep body temperatures during rewarming after hypothermia, is thought to endanger the heart by further cooling from cold blood presumed to be returning from the periphery. However, afterdrop is not always observed, depending on the circumstances. To explore this phenomenon, mild hypothermia was induced quantitatively with a suit calorimeter, using several patterns of cooling and rewarming. When cooling was rapid and followed immediately by rewarming, there were typical afterdrops in the temperatures measured in the rectum, auditory canal, and esophagus. However, when rewarming was delayed, or when cooling had been slow and prolonged, afterdrop was not seen. Afterdrops were then observed in two physical models that had no circulation: a bag of gelatin and a leg of beef. Central layers continued to give up heat as long as the surrounding layer was cooler. These results, together with recent findings by others that peripheral blood flow is low until afterdrop is complete, make this circulatory explanation of afterdrop improbable. Alternatively, afterdrop can be explained by the way heat moves through a mass of tissue.