Abstract: Vasoconstrictor .alpha.-adrenergic agonists of formula (I)A--B--C (I)wherein, A is a 2-imidazoline group or a guanidine group; B is a chemical bond or a linking group one or two atoms in length; and C is a C.sub.6-10 mono- or bi-cyclic group which is either an aromatic group, a heteroaromatic group containing only one hetero-atom, or a group containing an aromatic moiety; and which group C may be substituted by C.sub.1-4 alkyl, C.sub.1-4 alkoxy, halogen or hydroxyl; or salts thereof, are useful in the treatment and prevention of diarrhoea in livestock. Compositions of these compounds are described.