Wood frogs (Rana sylvatica) were frozen to -2.5 degrees C under five distinct cooling regimes to investigate the effect of cooling rate on survival. Frogs survived freezing when cooled at -0.16 degrees C.h-1 or -0.18 degrees C.h-1, but mortality resulted at higher rates (-0.30 degrees C.h-1, -1.03 degrees C.h-1, and -1.17 degrees C.h-1). Surviving frogs in the latter groups required longer periods to recover, and transient injury to the neuromuscular system was evident. Some of the frogs that died had patches of discolored, apparently necrotic skin; vascular damage, as indicated by hematoma, also occurred. It is concluded that slow cooling may be critical to the freeze tolerance of wood frogs. Additional studies examined the effect of cooling rate on physiological responses promoting freeze tolerance. Mean glucose concentrations measured in plasma (15-16 mumol.ml-1) and liver (42-45 mumol.g-1) following a 2-h thaw did not differ between slowly- and rapidly-cooled frogs but in both groups were elevated relative to unfrozen controls. Thus, freezing injury to rapidly-cooled frogs apparently was not mitigated by the presence of elevated glucose. Water contents of liver tissue, measured 2 h post-thawing, did not differ between slowly-cooled (mean = 77.6%) and rapidly-cooled (mean = 78.5%) frogs. However, the mean hematocrit of slowly-cooled frogs (48%) was significantly higher than that (37%) of frogs cooled rapidly, possibly owing to differences in the dynamics of tissue water during freezing.