The aim of the current study was to investigate: (1) secretion P-4, E-2 and A(4) by whole porcine uterine explants harvested during days 14-16 of pregnancy (implantation) and days 30-32 of pregnancy (placentation); (2) whether progesterone, estradiol, and other factors: oxytocin (OT), arachidonic acid (AA; substrate for prostaglandins synthesis) as well as forskolin (FSK; adenylate cyclase activator) and cAMP (cyclic adenosine monophosphate; second messenger) regulate the release of steroid hormones within uteri during implantation and placentation. Concentrations of P-4, E-2 and A(4) were measured in culture media by RIA. Basal uterine production of E-2 after 3-h incubation was about 2-fold higher on days 30-32 compared to days 14-16 of the pregnancy. Progesterone very strongly increased secretion of E-2 during the implantation and placentation after 3- and 24-h incubations. Control production of P-4 by uterus explants after 3-h incubation was higher during placentation than during implantation. Estradiol consistently increased the secretion of P-4 on days 14-16 and days 30-32 of the pregnancy, after 3- and 24-h incubation. The basal secretions of A(4) from uterine explants after 3-h incubation in both pregnancy periods were at the same level. Progesterone stimulated A(4) secretion on days 14-16 after 3- and 24-h. Estradiol also very strongly increased A(4) secretion during both implantation and placentation during the incubation periods. Oxytocin did not affect the release of E-2, P-4 and A(4) (p > 0.05) by tissue explants from either of the pregnancy periods. AA significantly stimulated E-2 secretion during implantation (after 3- and 24-h incubations) and placentation (after 3-h incubation). In contrast, AA did not effect the release of P-4 or A(4) in either of the pregnancy periods. FSK and cAMP significantly stimulated E-2 release during implantation after 3- and 24-h incubation, while during placentation only after longer incubation (24-h). The secretion of P-4 was also increased by FSK during implantation after 24-h incubation and during placentation (after 3- and 24-h incubation) as well as by cAMP in both pregnancy periods after 3- and 24-h. In addition, forskolin and cAMP very strongly stimulated A4 release with uterine explants after 3- and 24-h incubation at the time of implantation and placentation. In summary, this is the first demonstration that: 1) the production and release of E-2 and P-4 (but not A(4)) from the pig uterine explants increase with advancing gestation; and 2) P-4, E-2, arachidonic acid, forskolin and cAMP are involved in the regulation of uterine steroidogenesis, which seems to depend on the pregnancy period and incubation time.