24Na-labelled hypertonic saline solution was injected into the uterus by the intra-amniotic or the extra-amniotic route, and six hours later determinations were made of the 24Na content of the placenta, myometrium, and decidua. The tissue specimens were obtained by subtotal hysterectomy. The activity of 24Na proved to be high in the decidua and in the fetal part of the placenta, but low in the myometrium. During labour, large amounts of prostaglandins have been found in the decidua, but little or none in the placenta. The present findings therefore lend further support to the assumption that hypertonic saline solution provokes abortion by inducing release of lysosomal phospholipase A2 into the cytoplasm of damaged decidual cells, resulting in prostaglandin synthesis and liberation.