Abstract The effect of decaborane (B 10H 14) upon the 5-hydroxytryptamine (5HT) content and the aromatic l-amino acid decarboxylase activity in rat brain was studied. Also examined was the effect of decaborane upon the 5-hydroxytryptophan (5HTP)-induced increase in rat brain 5HT. Decaborane diminished the 5HT content of rat brain and also abolished the 5HTP-induced 5HT elevation. This boron hydride inhibited decarboxylase in a rat brain homogenate system prepared from animals injected with decaborane. Decarboxylase was also inhibited in control rat brain homogenates to which decaborane was added in vitro. From the data it was concluded that probably one of the mechanisms of action of decaborane is inhibition of decarboxylase, but a second component may be a release phenomenon.