This invention relates to drilling fluid additives that suppress clay swelling within a subterranean well and to methods for controlling clay swelling during the drilling of a subterranean well. Aliphatic polyamines and aliphatic acids react to form polyamides and polyamino acids that are water soluble, that have molecular weights of less than about 1000, and that have low toxicity levels. The polyamides and polyamino acids are added to water base drilling fluids which are circulated throughout a well.When X in the acid is a hydroxyl group, the polyamide products have the following general structure: ##STR1## When X in the acid is a halogen such as chlorine, the polyamino acid products have the following general structure: ##STR2## The drilling fluid additives and the method of controlling clay swelling provide for improved control of the rheological properties of drilling fluids along with an increased environmental compatibility.