Rat isolated hearts perfused with delta 1-THC (0.5 micrograms/ml) showed a reduction in the rate of beating which was not altered by pretreatment with propranolol (2 micrograms/ml), atropine (4 micrograms/ml) or hexamethonium (4 micrograms/ml). 2. Propranolol (2 micrograms/ml) also caused a decrease in the rate of beating, which was not affected by pretreatment with delta 1-THC (0.5 micrograms/ml). 3. In pithed rats, propranolol (2 mg/kg, i.v.) caused a decrease in the pulse rate, which was not altered by prior administration of delta 1-THC (1mg/kg, i.v.). 4. In both preparations, the responses to isoprenaline were markedly reduced or abolished by propranolol, but they were unaffected by delta 1-THC. 5. It is concluded that the hypotensive and cardiac slowing actions of delta 1-THC are not mediated by activation of parasympathetic nerves or by beta-adrenoceptor blockade.
