SUMMARY1. Using free-flow micropuncture techniques, the rate of reabsorption from the proximal convoluted tubules was measured in virgin, 19-day-pregnant and 6-to 7-day-lactating Sprague-Dawley rats.2. In separate groups of animals proximal convoluted tubules were filled with silicone rubber, dissected out and their lengths measured.3. Single nephron glomerular rate was increased in pregnant and lactating animals. 4. The rate of reabsorption per unit length of the proximal convoluted tubule was decreased in pregnant animals but similar in virgins and lactating rats.5. The proximal convoluted tubule was increased in length in pregnant and lactating animals. As a result the total reabsorption by the proximal convoluted tubule was similar in virgin and pregnant animals but increased in lactating animals.6. The amount reabsorbed by the loop of Henle and distal parts of the nephron is greater in pregnant and lactating animals than in virgins.