Erythrocyte osmotic fragility and plasma ionic composition were studied in rats subjected to acute hypothermia. A decrease in osmotic fragility and a significant increase in plasma magnesium and total phosphorus were observed in blood from hypothermic rats in relation to control. A decrease in erythrocyte osmotic fragility from hypothermic animals was observed when the test was performed at 37 degrees C, whereas osmotic fragility was unaltered if the test was carried out at body temperature. This could be interpreted as an adaptative response to counteract the opposite effect on erythrocyte osmotic fragility observed at low temperature 'in vitro'.