Human decidual cells are known to produce 1,25-(OH)2D3 at the end of pregnancy, the present study evaluates this capacity, and the part played by stromal decidual cells, in early pregnancy. Cells were obtained from nine human decidua by aspiration or curettage during early pregnancy (7-10 weeks), separated on Ficoll-Paque and plastic adherence, and incubated for 1 h with 25-(OH)D3. Incubation medium and cells were extracted and chromatographed on two successive HPLC systems. The cells examined were of both physiological and pathological (ectopic pregnancy) origin. Endometrial cells obtained in four non-pregnant situations (myomas) were also studied to determine whether the 1,25-(OH)2D3 synthesis by the uterus is associated with the appearance of decidual cells. Results show that human decidual cells from early pregnancy convert 25(OH)D3 (2.5 nM or 2.5 microM) into a metabolite with the physicochemical characteristics of synthetic 1,25-(OH)2D3. This ability is shared by cells isolated during early pregnancy, whether physiological or ectopic (tubal pregnancy). Non-adherent cells, which include mainly stromal decidual cells, are less able to produce 1,25-(OH)2D3 than are the adherent cells, suggesting that macrophages, granulocytes or as yet unidentified cell types are required for the 1,25-(OH)2D3 production by decidual tissue during early human pregnancy. In addition, one out of four experiments with non-pregnant endometrial cells could produce 1,25-(OH)2D3 suggesting that, although not the rule in the non-pregnant state, in vitro production of 1,25-(OH)2D3 by uterine cells can be found in the absence of decidual cells.