The influence of methotrexate (MTX), dibutyryl cyclic AMP, and actinomycin D on production of human chorionic gonadotropin (HCG) in normal first trimester human placental organ cultures was compared. Actinomycin D (10(-8) to 10(-6) M) elevated HCG production by as much as 3.5-fold in normal placenta, and a 2-fold increase in HCG levels was obtained by treatment with dibutyryl cyclic AMP (1 mM) and theophylline (1 mM). The combination of dibutyryl cyclic AMP (1 mM) plus theophylline (1 mM) and actinomycin D (10(-8) M) additively enhanced HCG production by 4.5-fold. In contrast, HCG levels in normal placental organ cultures were unaffected by MTX (10(-8) to 10(-5) M) despite several differing treatment regimens. The JAr line of human choriocarcinoma cells, on the other hand, exhibited an 8-fold increase in HCG levels following MTX exposure (10(-7) M). Incorporation of selected radiolabeled precursors of the de novo and salvage pathways of DNA synthesis was evaluated to assess potential metabolic alterations underlying the differential HCG response of these cultures to MTX. Deoxyuridine incorporation into DNA was decreased similarly in both normal and malignant placenta following MTX exposure. However, deoxycytidine incorporation was inhibited by MTX in normal placental cultures but was elevated by as much as 4-fold in JAr cultures exposed to MTX. Thymidine incorporation into DNA was increased in both groups in the presence of MTX; however, thymidine incorporation was more profoundly stimulated (5-fold) in normal placenta than in JAr cultures (2.5-fold). These data indicate dissimilar utilization of the de novo and salvage pathways of DNA synthesis by these cultures which may explain their differential responsiveness to MTX.