During the main period of egg-shell formation in the fowl, calcium is removed from the blood at a mean rate of approximately 125 mg/h for an average shell containing 2 g calcium. This represents a 'clearance-time' of approximately 12 min for the total calcium in the circulation. It seems unlikely that a rate of absorption of this magnitude from the digestive tract can be sustained throughout the full period of rapid shell calcification (16 h) and the evidence suggests that some degree of bone mobilization takes place for at least part of the time during which shell formation occurs. For it to be effective, it is clear that a rapid mechanism for calcium mobilization is essential if a serious hypocalcaemia is to be avoided, and one mediated by the parathyroid gland seems probable. The aim of the present work was to determine the response-time of laying hens to injected bovine