The dynamics of the in vitro synthesis and release of mouse placental proteins and mouse placental lactogen (mPL) were examined by culturing explants from mouse placentae from days 11, 14, and 17 of gestation for up to 24 h. The amount of mPL in medium and tissue was determined by RIA, and new protein synthesis was measured by the incorporation of [3H]leucine into mPL and proteins. The rate of synthesis of 3H-labeled proteins and [3H]mPL was constant for 17 h of incubation, confirming the viability of the placental tissue. Rapid and preferential release of newly synthesized mPL was observed; 88% of the mPL synthesized in 24 h was released into the medium, and the ratio of [3H]mPL to 3H-labeled proteins in the medium (14-18%) was significantly higher than that in the tissue (1-4%). There is not a stable storage pool of mPL under these conditions, as demonstrated by the similarity of the ratios of counts per min of [3H]mPL to nanograms of mPL in the medium and tissue. This conclusion is supported by the observed parallelism of mPL synthesis and release and the low tissue content of mPL. Release of mPL per mg tissue was similar for day 11 and day 14 placentae. It appears that mPL release in the absence of in vivo regulatory factors is strictly a function of placental mass at this time in gestation. A lower release from cultured day 17 placentae probably represents a decrease in the level of metabolic activity of the placenta as parturition approaches.