Prostaglandin (PG) E2 was the major PG released from the superfused guinea-pig uterus on Day 7, followed by in descending order 6-oxo-PGF1 alpha, thromboxane (TX) B2 and PGF2 alpha. However, the outputs of all four substances were low and were very similar. By Day 15, PGF2 alpha output from the superfused uterus had increased 21.9-fold, whereas the outputs of PGE2, 6-oxo-PGF1 alpha and TXB2 had increased only 1.8-, 2.9- and 1.2-fold, respectively. A mechanism is apparently "switched on" between Days 7 and 15 which causes a fairly specific increase in the release of PGF2 alpha from the uterus. Progesterone and/or estradiol had no effect on PG or TX release when superfused over the uterus on Day 7, nor did they have any effect on PG and TX release from the Day 15 uterus when administered separately. When administered together, however, they significantly inhibited PGF2 alpha, PGE2 and 6-oxo-PGF1 alpha, but not TXB2, release from the Day 15 uterus. Oxytocin had no effect on PG release from the Day 7 or Day 15 uterus, while A23187 stimulated PGF2 alpha, 6-oxo-PGF1 alpha and, to a lesser extent, PGE2 release from the uterus on both Days 7 and 15. Oxytocin is apparently not important for stimulating PGF2 alpha release from the guinea-pig uterus in relation to luteolysis, whereas increasing intracellular free Ca++ levels may be part of the mechanism for "switching on" uterine PG synthesis. Furthermore, changes in intracellular free Ca++ levels in the endometrium may be responsible for the pulsatile nature of PGF2 alpha release from the uterus.