The effects of a crude extract of the stem bark of Cassia abbreviata o
n mean arterial pressure in anaesthetized rats were investigated. Bolu
s injections of the stem bark elicited a transitory fall in blood pres
sure which was dose-dependent. The depressor response was not blocked
by atropine nor propranolol thus indicating that muscarinic and beta 2
receptors are not involved. However it was partially blocked by diphe
nhydramine, and H1 antagonist. The latter also partially blocked the d
epressor response to histamine which is mediated partially through bot
h H1 and H2 receptors. It is concluded that the stem bark of C. abbrev
iata contains histamine or a chemically related substance and that it
is this component which is responsible for lowering the blood pressure
.