Abstract: The invention provides compounds of the formula ##STR1## wherein Y is --CH.sub.2 -- or --CO--;R.sub.1 is F, Cl or OH; R.sub.2 is H, F or Cl; and R.sub.3 is H, CH.sub.3 or CH.sub.2 CH.sub.3, excluding 4-(5-chloro-2,3-dihydro-1H-inden-2-yl)-1H-imidazole and 4-(4-chloro-2,3-dihydro-1H-inden-2-yl)-1H-imidazole; and their non-toxic acid addition salts and mixtures thereof. Processes for the preparation of these compounds are described, as are pharmaceutical compositions comprising at least one of the compounds or its salt. The compounds and their non-toxic salts exhibit valuable pharmacological activity and are highly selective and long acting antagonists at .alpha..sub.2 -adrenoceptors. Their peroral bioavailability is good. The compounds are especially useful in the treatment of cognitive disorders.