In the present study we examined the effect of depletion of central nervous system serotonin by 5,7-dihydroxytryptamine on blood pressure in male Wistar rats. We also analyzed the relationship between the serotonergic and renin-angiotensin systems. Blood pressure was determined before and after intracisternal administration of 5,7-dihydroxytryptamine, 200 micrograms in saline with 1 mg/ml ascorbic acid (n = 9). Control rats (n = 8) received intracisternal vehicle. Before sacrifice, blood and cerebrospinal fluid samples were obtained. The brain was dissected in several areas. Serotonin, norepinephrine, angiotensinogen, and reninlike concentrations were determined in the brain parenchyma; angiotensinogen concentration was evaluated in cerebrospinal fluid and plasma samples; plasma renin activity was also measured. Treatment produced a significant decrease in blood pressure (-10 mm Hg; p less than 0.025) and, simultaneously, a high depletion of serotonin stores in the studied central areas (p less than 0.001), except in the cerebral cortex. Reninlike concentration was increased in the medulla oblongata (p less than 0.005) and the brainstem (p less than 0.02) after 5,7-dihydroxytryptamine treatment. Angiotensinogen concentration was decreased in the hypothalamus and elevated in the spinal cord. Angiotensinogen concentration in cerebrospinal fluid, plasma angiotensinogen concentration, and plasma renin activity did not change with treatment. Serotonin concentration in the cerebrospinal fluid remained unchanged, while the 5-hydroxyindoleacetic acid level was diminished (-47%; p less than 0.001). Intracisternal administration of 5,7-dihydroxytryptamine produced a hypotensive effect in normal rats and several modifications of the renin-angiotensin complex, suggesting a relationship between the monoaminergic and peptidergic systems.