THERE are two major angiotensin II receptor isoforms, AT(1) and AT(2). AT(1) mediates the well-known presser and mitogenic effects of angiotensin II (refs 1-5), but the signalling mechanism and physiological role of AT(2) (refs 6-11) has not been established. Its abundant expression in fetal tissues(12) and certain brain nuclei(13) suggest possible roles in growth, development and neuronal functions. Here we report the unexpected finding that the targeted disruption of the mouse AT(2) gene resulted in a significant increase in blood pressure and increased sensitivity to the presser action of angiotensin II. Thus AT(2) mediates a depressor effect and antagonizes the AT(1)-mediated presser action of angiotensin II. In addition, disruption of the AT(2) gene attenuated exploratory behaviour and lowered body temperature. Our results show that angiotensin II activates AT(1) and AT(2), which have mutually counteracting haemodynamic effects, and that AT(2) regulates central nervous system functions, including behaviour.