The interactions of hormones with plasma membranes in the human placenta have not been characterized for specific maternal and fetal components. The human placenta is exposed to maternal blood at the microvillus brush border membrane (MVBB) and to fetal blood at basal plasma membranes (BPM). In order to clarify possible hormone membrane interactions at these two sites, hormone sensitive adenylate cyclase (AC), insulin receptor and marker enzyme analyses were compared in MVBB and in BPM from 10 term human placentas. MVBB was 12 fold enriched in alkaline phosphatase, 5′nucleotidase, Mg-ATPase and in specific insulin binding. Adenylate cyclase could not be demonstrated in the MVBB. Basal plasma membranes, prepared after removal of the MVBB, were rich in A&Cbd;; basal activity was 46±5.6pmolesmg-1 min-1 ≥ and sensitivity was demonstrated for epinephrine, fluoride, prostaglandins E1, and F2α. Enzyme analysis of this fraction showed no enrichment with MVBB markers or insulin receptors. In the MVBB, insulin receptor concentration was 242±42nmoles 10-6 mg-1 compared to 20.7±2.9 in the BPM (p<.01). Alkaline phosphatase was 3.02±.43 in MVBB compared to .30±.032 in the BPM (p<.01). Micro-villous plasma membrane is rich in insulin receptors suggesting interaction with maternal insulin but does not contain adenylate cyclase. Basal membrane, presumably exposed to fetal hormonal influence, is rich in adenylate cyclase but not in insulin receptors.