We have used stimulation of the activity of the brain type creatine kinase (CK) isoenzyme as a response marker to examine the effects of vitamin D metabolites, PTH, and calcitonin in cultured explants of placenta, decidua, and amnion from normal human deliveries. We found a biological response to PTH in placenta and amnion and to vitamin D metabolites in all three tissues. In the amnion, CK activity increased 2.3-fold after 24 h of incubation in 2.5 nM 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], 3.8-fold when incubated with 12.5 nM 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3] and 2.7-fold when incubated with 10 U/ml bovine PTH. In the decidua, 24,25-(OH)2D3, but not 1,25-(OH)2D3 or bPTH caused a 1.7-fold increase in CK activity. In contrast, the placenta responded to 1,25-(OH)2D3 with a 1.6-fold increase in CK activity and to bPTH, with a 1.7-fold increase but did not respond to 24,25-(OH)2D3. Bovine calcitonin (100 ng/ml) had no effect on CK activity in any of the three tissues. Nearly all CK in both the unstimulated and stimulated explants was the brain type isoenzyme. CK activity increased significantly between 1 and 4 h after hormonal treatment in all experiments. The enzyme activity rose steeply with dose and reached a significant increase, and usually a plateau, at hormone concentrations considered to be physiological in vivo. [3H]Thymidine incorporation into DNA increased in parallel to stimulation of CK activity in all experiments, except that PTH did not increase DNA synthesis in the placenta. PTH did cause an increase in cAMP production in explants of amnion (1.5-fold) and placenta (2.6-fold).