1. Eighty-two single determinations of ammonia and urea excretion by Xenopus laevis indicated that the percentage of ammonia varied from 40 to 80%, with a mean value of 62%.
2. Measurements of excretion on successive days after feeding showed that a large amount of ammonia was produced soon after feeding, but that ammonia excretion declined rapidly. Urea excretion, not so high initially, remained more or less constant until the third or fourth day, often exceeding ammonia excretion at that time. Thereafter, it also declined and the excretion of both substances reached a constant starvation level by the fifteenth day.
3. Both ammonia and urea excretion were equally affected by temperature. The Q10's were near 2 in the range 20-30° C., but greater in the range 10-20° C.
4. At least 86% of ammonia, and 81% of urea were excreted through the cloaca.
5. The mean 24 hr. urine output of Xenopus at 20% C. was 23.6 ml. per 100 g. body weight.
6. Although the blood ammonia concentration did not appear to be zero, the urine/blood concentration ratio of ammonia was greater than 100. The urine/blood concentration ratio of urea was not significantly different from unity, and constant over a very wide range of concentrations.
7. The above result is interpreted to indicate passive glomerular filtration of urea, and little or no tubular reabsorption of water.
8. It is suggested that ammonia is formed in the kidney, and actively secreted into the glomerular filtrate.