The effects of phenylalanine (PHE) and tyrosine (TYR) on cold acclimation were studied in mice of the ddY strain. At 5 weeks of age, mice were maintained at 4 degrees C for 4 weeks. Test groups of mice were supplied with a 0.05% (w/v) solution of PHE or TYR as drinking water in addition to water. We measured changes in body weight, intake of water and food, rectal temperature upon acute exposure to -20 degrees C, weight of interscapular brown adipose tissue (BAT) and the levels of glucose, nonesterified fatty acids (NEFA) and 3H-butyrate in the blood prior to and after exposure to the lower temperature of -20 degrees C. Chronic exposure to cold (4 degrees C) reduced body-weight gain for the first two weeks but weight gain recovered within the next two weeks. PHE and TYR partially inhibited the gain in body weight under exposing to cold. TYR reduced the gain of body weight under room temperature. Exposure to cold stimulated the daily consumption of food. Both PHE and TYR somewhat enhanced the food intake when exposed to cold. Exposure to cold rendered mice resistant to severe cold (-20 degrees C). Both PHE and TYR did not reduce this resistance except in the early state in the case of TYR. Exposure to cold increased the weight of BAT but both PHE and TYR prevented this increase. The effect of TYR was determined in the case fed under room temperature. Cold exposure changed the utilization of glucose, NEFA and 3H-butyrate in the blood when exposed to severe cold (-20 degrees C). Both PHE and TYR prevented the change in NEFA. It remains to be confirmed whether the growth of brown adipocytes is always necessary when mice acclimate to cold.