Compounds of the Formula: ##STR1## wherein: R.sup.1 and R.sup.2 are independently hydrogen or lower alkyl; R.sup.3 is hydrogen, lower alkyl, lower alkoxy, hydroxy, halo, optionally substituted phenyl, amino, nitro or trifluoromethyl; and X is --NH--, --O-- or --S--; and the pharmaceutically acceptable salts thereof exhibit high affinity and selectivity for imidazoline receptors, and are are particularly useful for modification of the female reproductive cycle.