1. The pharmacological properties of F-180, a vasopressin (VP) structural analogue, were determined on CHO cells expressing the different human vasopressin and oxytocin (OT) receptor subtypes. Binding experiments revealed that F-180 exhibited a high affinity for the human V(1a) receptor subtype (K(i)=11 nM) and was selective for this receptor subtype. 2. Functional studies performed on CHO cells expressing human V(1a) receptors indicate that similarly to AVP, F-180 can stimulate the accumulation of inositol phosphate. The activation constant (K(act)) for both F-180 and AVP was 1.7 nM. F-180 was also an agonist for the human V(2) and V(1b) receptor subtypes and an antagonist for the human OT receptor. 3. Since marked species pharmacological differences for vasopressin receptors have been described, we studied the properties of F-180 on various mammalian species. F-180 showed high affinity and good selectivity for human and bovine V(1a) receptors, but weak affinity and non selective properties for rat V(1a) receptors. 4. To assess the functional properties of F-180 on a native biological model, we performed studies on primary cultures of cells from bovine zona fasciculata (ZF). As AVP, F-180 stimulated inositol phosphate accumulation and cortisol secretion with similar efficiency. 5. In conclusion, we demonstrate that F-180 is the first selective V(1a) agonist described for human and bovine vasopressin receptors. Therefore F-180 can be used as a powerful pharmacological tool to characterize the actions of vasopressin that are mediated by V(1a) receptor subtypes.