In one-hour intervals before feeding and 1, 2, 3, 4 and 5 hours after feeding, three wethers were studied for the quantitative secretion of urea N and ammonia N into the caecum pouch (volume 80 ml). The amount of ammonia N secreted into the caecum ranged from 10.0 +/- 2.8 to 20.0 +/- 3.5 microM/h. The concentration of ammonia N was 271.2 +/- 60.6 microM/l, on the average. The passage of urea N showed no statistically significant differences before and after feeding (37.8 +/- 4.2 - 41.4 +/- 5.0 microM/h). The concentration of urea N in the caecum pouch was from 617.8 +/- 67.8 microM/l to 710.7 +/- 82.1 microM/l. Urea penetrates through the caecum wall into the caecum mostly in the form of a whole molecule, only its small proportion has the form of ammonia, its hydrolysis product.