SUMMARY In pregnant and in lactating goats the rate of cortisol secretion was measured by continuous intravenous infusion of tritium-labelled cortisol. Uptake of cortisol by the mammary gland was measured by simultaneous sampling of blood from the carotid artery and superficial epigastric vein, and measurement of mammary blood flow. In pregnant goats the rate of cortisol secretion was 9 μg/min but in preparturient and in lactating goats the rate was about 32 μg/min. There was a small but significant difference in cortisol concentration between the carotid artery and mammary vein. In pregnant goats the mammary uptake of cortisol was about 0·2 μg/min; this was increased to about 1·3 μg/min in preparturient and in lactating animals. Negligible quantities of cortisol were secreted in the milk. Cortisol was administered by continuous infusion for 4 h into two lactating goats. Although there was some stimulation of milk secretion it seems likely that it resulted from a systemic rather than local action of cortisol.