Abstract In this study arginine vasopressin (AVP) and oxytocin (OT) receptors have been characterized in the brushtail possum. AVP receptors were characterized using [³H]AVP and the radioiodinated AVP V_1a receptor antagonist ¹²⁵I-labelled [(C₆H₅-CH₂CO)-O-methyl-d-Tyr-Phe-Gln-Asn-Arg-Pro-Arg-Tyr- NH₂] while OT receptors were characterized using the radioiodinated OT receptor antagonist ¹²⁵I-labelled d(CH₂)₅[Tyr(Me)²,Thr⁴,Orn⁸, Tyr-NH₂⁹]-vasotocin. The receptor affinities and densities have been compared with the rat AVP and OT receptors. Low densities of OT receptors were present in the possum ovary and kidney. High densities of AVP-binding sites were found in the possum adrenal, testis, mesenteric artery, ovary and renal medulla and lower densities in the possum liver. The AVP-binding sites showed marked differences in ligand-binding characteristics from the rat AVP V_1a and V₂ receptors. Receptor affinities were similar between tissues, except for a distinctly lower value in the renal medulla. It is concluded that the brushtail possum expresses AVP receptors with distinct ligand specificities from those of the rat AVP V_1a and V₂ receptors. Journal of Endocrinology (1995) 144, 19–29