In the present study, we used an explant culture system of the human decidual tissues involving a new sampling method for investigating the productivity of immunoreactive prolactin (IR-PRL) for a 5 day period in labored and nonlabored deliveries. The maximal release of IR-PRL in the incubation medium for each 24 hour interval was achieved from the 2nd to the 3rd day of culture in both groups, which was 145.2 +/- 14.0 ng/ml/10 mg w.w. (mean +/- s.e.; n = 7) in the labor group and 101.5 +/- 16.3 ng/ml/10 mg w.w. (mean +/- s.e.; n = 4) in the nonlabor group. The release of IR-PRL in both groups was not significantly different for the first 3 days. However, the amount of IR-PRL released in the nonlabor group was 50 to 70% of that of the labor group. The tissue content of IR-PRL in both groups ranged between 3.0 and 5.0 ng/ml/10 mg w.w. From these results, it was concluded that 1) our explant culture system for the human decidual tissues produced considerably more IR-PRL than those previously reported and 2) productivity of IR-PRL was lower in nonlabored delivery than in labored delivery and 3) since the tissue content of IR-PRL for each 24 hour interval was very small, it should be strongly emphasized that the production and release of IR-PRL takes place simultaneously in the human decidual tissues.