1. In freely moving, unanaesthetized rats bile flow was measured continuously over the whole day-night cycle. Bile composition was analysed and the influence of food intake on bile flow was investigated.

2. In both sexes a distinct circadian variation of bile production was observed. The mean night-time production was 50% higher than the day-time value for female rats and 38% for male rats. In the morning when the light was switched on, a sharp decrease in secretion rate was prominent and bile flow gradually increased in the afternoon.

3. The pattern of food intake was positively correlated with the pattern of bile secretion. During fasting only the general level of bile flow decreased, but the circadian variation persisted. Refeeding again increased the mean level of bile flow.

4. The chenodeoxycholate/cholate ratio in these rats with permanent bile fistulae was higher than in rats with ‘acute’ bile fistulae and changed during the day-night cycle. The ratio decreased from 1.01 at 05.00 hours to a minimum of 0.41 at 15.00 hours.

5. During the day-night cycle the sodium, potassium, calcium and cholesterol concentrations were relatively constant. The total bile salt concentration was only slightly changed, so that both the bile salt-dependent fraction and the bile salt-independent fraction were subject to about the same circadian variations.

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