Freeze tolerance is a complex cold-hardiness adaptation that has independently evolved in a diverse group of organisms, including several ectothermic vertebrates. Because little is known about the mechanistic basis for freeze tolerance in reptiles, we compared responses to experimental freezing in winter-acclimatized hatchlings representing nine taxa of temperate North American turtles, including ones that tolerated freezing and others that did not. Viability rates of hatchlings frozen to -3 degrees C for 72 h ranged from 0 to 100%. Tolerance to freezing was poor in Sternotherus odoratus, Graptemys geographica and Trachemys scripta, intermediate in Chelydra serpentina, and high in Emydoidea blandingii, Chrysemys picta bellii, C. p. marginata, Malaclemys terrapin, and Terrapene ornata, and generally reflected the winter thermal ecology of each taxon. Plasma activity of lactate dehydrogenase (LDH), a novel in vivo index of freeze/thaw damage, corroborated viability assessments and demonstrated that cryoinjury occurred even in surviving turtles. Irrespective of taxon, cryoinjury tended to be higher in smaller individuals and in those having relatively low water contents; however, bases for these associations were not apparent. Screening for certain organic osmolytes that might promote freezing survival by colligatively reducing ice content and limiting cell dehydration showed that the plasma of unfrozen (control) turtles contained small quantities of glucose (1.3-5.8 mmol l(-1)) and lactate (0.6-3.2 mmol l(-1)) and modest amounts of urea (range of mean values for all taxa 8.2-52.3 mmol l(-1)). Frozen/thawed turtles of all taxa accumulated modest amounts of glucose and lactate that jointly raised the plasma solute concentration by 30-100 mmol l(-1). We conclude that organic osmolytes accumulated both before and during freezing may promote survival in species that have evolved a tolerance to freezing, but are not necessarily accumulated for that purpose.