The steady-state level of histamine (HA) in the hypothalamus of portoc
aval shunted (PCA) rats was significantly higher than control values,
while the concentration of N(tau)-methylhistamine (t-MeHA) was unaffec
ted. Two hours after administration of the monoamine oxidase inhibitor
, pargyline (PG, 80 mg/kg i.p.), the tissue concentration of t-MeHA wa
s increased and the net increase was almost the same for PCA and sham
operated rats. It appears, therefore, that there is no difference in t
he activity of histaminergic neurons between PCA and control rats.