Twenty-eight female mice of the first filial generation of a C57BL KR cross were used. Twelve mice developed tumours in the thoracoabdominal region 11 months after birth. A drastic enhancement of alcohol preference occurred within one month after the discovery of mammary adenoma and the animals eventually became drunk and remained so for several days. Of 12 mice developing tumours, 10 showed a conspicuous degree of drunkenness and decrease of body weight, with some developing tremors and wet dog shakes reminiscent of the alcohol withdrawal syndrome. The remaining 2 also displayed a marked preference for alcohol, though not to the extent of developing the drunkenness syndrome. None of 16 mice in the tumour-free group was observed to have significant alcohol preference. Most animals in the tumour-free group ingested alcohol mainly at night, whereas those in the tumour-bearing group ingested alcohol throughout the 24 hours. The blood alcohol concentrations of the drunken mice were over 4 mg/ml.