Abstract 1. 1. The glomerular filtration rate (GFR) in colostomised laying hens decreased significantly during active shell formation. This reduction may play a role in the renal conservation of calcium during this period. 2. 2. Renal plasma flow (RPF) increased significantly during shell dormation. Calculations based on the data for renal plasma flow revealed that during active shell formation the percentage cardiac output supplying the kidneys doubled. 3. 3. The decrease in GFR and increase in RPF were both abolished in colostomised laying hens infused with calcium during egg shell formation. 4. 4. It is suggested that the hyper-secretion of parathyroid hormone (PTH) during shell formation may lead to a decrease in GFR and the redistribution of blood in the renal peritubular circulation, thereby facilitating the tubular secretion of phosphorus.