The invention relates to a process for the preparation of a compound of formula (I) ##STR1## wherein R represent a hydrogen atom or a methyl or ethyl group, which comprises cyclising a compound of formul (IV) ##STR2## Where R represents a hydrogen atom, the compound of formula (IV) may optionally be alkylated to produce a compound in whcich R is methyl or ethyl. The compound of formula (I) is useful as an intermediate in the preparation of 2,3,4,5-tetrahydro-2-[(imidazol-4(or 5)-yl)methyl]-1H-pyrido[4,3-b]indol-1-ones having 5-HT.sub.3 antagonist activity.