We monitored the resumption of physiological functions in frogs that were frozen at -2 to -3 degrees C for 24 h and thawed rapidly (at 23-25 degrees C) or slowly (at 6-8 degrees C). Bodily functions were restored sooner during fast thawing, but this did not enhance the survival of frogs. The first physiological parameter to return was cardiac function, but during the early stages of thawing heart rates were lower than heart rates of unfrozen frogs at comparable body temperatures. Heart rates increased thereafter in conjunction with the rise in frog body temperatures. Spontaneous breathing and hindleg reflexes resumed after cardiac function, but neither response was exhibited by all frogs after the conclusion of the observation periods (3-4 h). Finally, isolated gastrocnemius muscles that had undergone in vitro freezing showed no significant (P greater than 0.05) impairment of twitch and tetanic tensions even as soon as 1 h after the onset of thawing. Body systems thus vary in their rates of recovery after nonlethal freezing episodes. Furthermore, recovery of specific body systems corresponds to essential needs that must be met immediately after thawing, such as reperfusion of body tissues.