A compound of formula I, ##STR00001## wherein X, Z, R.sub.1 to R.sub.10, R.sub.15, R.sub.16, m, n, r and t are as defined in claim 1, or a pharmaceutically acceptable salt or ester thereof, useful as an alpha-2 antagonist. The compounds of formula I can be used for the treatment of diseases or conditions where antagonists of alpha-2 adrenoceptors are indicated to be effective.