Because adult rats have very low levels of the intestinal enzyme lactase, the ingestion of appreciable quantities of the disaccharide lactose may cause gastrointestinal distress. The present experiment was designed to demonstrate that adult rats will learn to avoid previously neutral stimuli which have been paired with lactose ingestion. Adult rats ingested both a novel solution [either tap water (WA) or 0.1% saccharin (SA)] and a novel food substance (49% powdered lab chow + 50% added disaccharide + 1% saccharin) during a single conditioning session. The added disaccharide was either sucrose (group SU-SA), lactose (groups HL-SA and HL-WA), or equal amounts of these two disaccharides (group LL-SA); a fifth group (LC-SA) consumed a sucrose-containing diet to which lithium chloride was added (5 mg per 1 g of diet). Separate feeding tests and drinking tests, carried out over several weeks, were used to assess the extent of conditioned taste avoidance. In the four feeding tests, rats were allowed to ingest powdered lab chow with added saccharin (but without added disaccharide), while in the four drinking tests, rats chose between tap water and a 0.1% saccharin solution. Group HL-SA demonstrated a substantial conditioned avoidance in both feeding and drinking tests, but group HL-WA showed avoidance only in feeding tests. Conditioned avoidance was weak in group LL-SA; the strongest avoidance was observed in lithium chloride-treated rats (group LC-SA). Results are related to previous research and to the hypothesis that a learned avoidance of milk may facilitate the weaning process in mammals.