  We define quasi--locally presentable categories as big unions of coreflective
subcategories which are locally presentable. Under appropriate hypotheses we
prove a representability theorem for exact contravariant functors defined on a
quasi--locally presentable category taking values in abelian groups. We show
that the abelianization of a well generated triangulated category is
quasi--locally presentable and we obtain a new proof of Brown representability
theorem. Examples of functors which are not representable are also given.
