Prostanoids are thought to take a prominent part in gestation and parturition physiology. Experiments were designed to determine the chronological alterations in placental synthesis and metabolism of prostanoids during the third trimester of gestation in the guinea-pig. Placental obtained at days 50, 55, 60, at term and after delivery were assayed (RIA and/or GC/MS) for PGE2, PGF2 alpha and TxB2 before (in vivo levels) or after 1 hour incubation (in vitro levels) for PGE2, PGF2 alpha, PGD2, TxB2, 6-keto-PGF1 alpha and PGF1 alpha. In addition, PGE2 and PGF2 alpha metabolism was measured by radiochromatography assay. No differences with gestational age were found in PGE2 and TxB2 in vivo levels but PGF2 alpha showed a slight increase around day 60. In contrast, in vitro levels of PGF2 alpha, TxB2 and PGE2 (in decreasing order of production) exhibited a marked increase from day 50 to delivery, whereas the other prostanoids showed no changes. Moreover, PGE2 and PGF2 alpha metabolism by the 15-hydroxyprostaglandin dehydrogenase was found to decrease during the same gestational period. These results are consistent with the idea that placenta in guinea-pig may be an important source of endogenous prostanoids and may contribute to the augmented intrauterine availability of prostanoids near parturition. The implications of these findings in relation to the mechanisms controlling synthesis and metabolism of prostanoids are discussed.