The activities of two phosphate-dependent glutaminase reactions characteristic of adult rat liver and kidney were determined in these organs from 2(1/2) days before to 7 days after birth and compared with the activities in the adult tissues. In the kidney, before and after birth, only the kidney type of activity was detected, and it increased in concentration in parallel with the steady growth of that organ throughout the period examined. In the liver, however, the kidney type of activity was the only one present 2(1/2) days before birth, and its concentration decreased to barely significant values by the end of the first week after birth. In contrast, the liver type of activity appeared only just before birth and increased to 60% of adult values over the next 4 days. There was no obvious relation between these changes in glutaminase type and changes in liver weight, protein content and total cell number that occurred during this time. But there was a very close correlation between the fall in kidney-type activity and the estimated fall in haematopoietic-cell number in liver.