Acylation of the o-aminonitrile (enaminonitrile) derivatives of certain pyrimido- [1, 6-a] indole, with acid anhydrides or acid chlorides was achieved yielding a tetracylic structure, pyrimido- [4',5' : 4,5 ] pyrimido [1,6-a] indole. The chlorine atom in the chloroalkyl moiety was replaced by different amines or condensed with thiourea and the formed salts were hydrolyzed to give the mercapto alkyl compounds. Moreover, enaminonitriles were reacted with oxalyl chloride to afford the 3-chlorocarbonyl derivatives from which esters and amides were prepared. The 3-carboxylic acid derivtives were also prepared and subjected to decarboxylation. Some of the prepared compounds were tested for their pharmacological activties.