Departments of Physiology, *The Middlesex Hospital Medical School, Cleveland Street, London, W1P 6DB, and St Mary's Hospital Medical School, Praed Street, London, W2 1PG
(Received 13 October 1976)
Recently there has been increasing interest in the possible role of foetal neurohypophysial hormones in parturition (Alexander, Bashore, Britton & Forsling, 1974; Burton, Illingworth, Challis & McNeilly, 1974). The significance of plasma levels in the mother and foetus depends on the placental permeability to the hormone. It has been suggested that in the sheep there is no placental transfer of oxytocin (Forsling, Jack & Nathanielsz, 1975) or arginine-vasopressin (AVP) (Alexander et al. 1974). Burton and her co-workers, however, reported that the guinea-pig placenta allows passage of oxytocin. It is therefore of interest to know whether this placenta which is haemochorial in nature is also permeable to AVP.
The placentae of nine guinea-pigs in the third trimester of pregnancy were perfused by recirculation