Departments of Obstetrics, Gynaecology and Reproductive Physiology, St Bartholomew's Hospital Medical College and the London Hospital Medical College, London, EC1A 7BE, and Hackney Hospital, London, E9 6BE (Received 18 September 1975) Levels of prolactin in the circulation show wide short-term fluctuations (McNeilly & Chard, 1974; McNeilly, Sturdy, Evans & Chard, 1974) suggesting episodic release. Prolactin levels were measured in closely spaced serial samples in an attempt to estimate the frequency of release. Informed consent was obtained from eight pregnant women, five women in the first week after delivery, and four non-pregnant women. Blood was collected through a Polythene cannula inserted 30 min before sampling. Three sampling schedules were used: (1) every 20 s for 10 min and then every minute for a further 10 min; (2) every 30 s for 15 min and then every minute for 15 min; (3) every minute for 30 min. Plasma prolactin was measured by