In a model of mouse embryo fibroblasts in culture, which is characterized by a very rapid decrease in rate of cell proliferation during early passages, we determined the production of prostanoids either by following the transformation of the radioactive precursor [14C]-arachidonic acid or by radioimmunoassays. Our results demonstrate that mouse embryo fibroblasts in culture produce spontaneously substantial amounts of PGE2 and 6ketoPGF1 alpha as well as trace amounts of PGF2 alpha and of lipoxygenase derivatives (Hetes). We investigated the stability of the different types of prostaglandins produced. We showed that 6ketoPGF1 alpha and PGF2 alpha were stable in vitro, whereas PGE2 as a consequence of its solubilization in an aqueous medium was metabolized by 50% over a 24 h period, independently of the presence of the cell, thus leading to a constant underestimation of real PGE2 concentration. However, comparison of the patterns of prostaglandin production among subcultures of different orders was possible, and showed that the total amount of prostaglandin produced as well as the relative proportions are fairly identical during the first three passages, although the cell proliferation pattern rapidly decreases among the serial subcultures. These results suggest that prostaglandin production does not represent in our experimental model an autocrine means of regulating cell growth.