The effects of histamine and some histamine-like agonists on blood pressure in the cat.

1. The effects of histamine on blood pressure have been compared with the effects caused by four histamine-like agonists in anaesthetized cats. 2. It has been confirmed that the depressor responses to histamine involve both H1- and H2-receptors: depressor responses also follow the administration of selective H1- and H2-receptor agonists. 3. 2-Methylhistamine, in doses up to 1 X 10(-7) mol/kg, lowers blood pressure by interaction with H1-receptors. Larger doses of 2-methylhistamine also lower blood pressure but this may involve H2-receptors. 4. 4-Methylhistamine, in doses up to 1 X 10(-7) mol/kg, lowers blood pressure by interaction with H2-receptors. Larger doses of 4-methylhistamine also lower blood pressure but this may involve H1-receptors. 5. 2-(2-Aminoethyl)pyridine and 2-(2-aminoethyl)thiazole both lower blood pressure by interaction with H1-receptors only. 6. The potential value and limitations of these compounds as tools to investigate the cardiovascular effects of histamine are discussed.