Abstract: The hydration of clays and shale in drilling operations may be inhibited by employing an aqueous based drilling fluid comprising a shale hydration inhibition agent comprising a polyaminopolyamide-epichlorohydrin resin, preferably the polyamino polyamide-epichlorohydrin resin is a reaction product of an aliphatic polyamine, an aliphatic polycarboxylic acid, and epichlorohydrin. The shale hydration inhibition agent should be present in the aqueous based drilling fluid in sufficient concentration to reduce the reactivity, such as swelling, of clays and shale when exposed to water-based drilling fluids.