The renin-angiotensin system of the mammalian brain seems to interfere with the process of cognition and has been associated with the hippocampal function in relation to mechanisms of learning and memory. In our investigation, the effects of angiotensin II (Ang II) and angiotensin IV (Ang II) on neuronal activity have been studied in the hippocampus of adult rats anesthetized with urethane. Excitatory effects of both angiotensins predominated over inhibitory effects. Angiotensins also induced an enhancement of burst discharges. These angiotensin-induced effects were blocked by the specific angiotensin antagonists. Our findings showed that the different effects of Ang II and Ang IV in behavioral studies are not similarly reflected in a different change of the discharge rate and/or pattern of hippocampal neurons after microiontophoretic administration of both substances.