Detection thresholds for corn starch, Polycose and sucrose in water were assessed by a new technique. Rats were trained to avoid these carbohydrates by giving them fluid containing carbohydrate mixed with lithium chloride on some days, and fluid containing sodium chloride with no carbohydrate on other days. Rats trained by this procedure avoided the substance that they were trained to avoid, but did not avoid other substances. Thus rats trained to avoid starch avoided mixtures containing as little as 0.025% starch, but did not avoid Polycose, sucrose or sodium chloride. Rats trained to avoid Polycose avoided fluids containing as little as 0.01% Polycose, but did not avoid fluids containing starch or sucrose. Finally, rats trained to avoid sucrose avoided fluids containing as little as 0.05% sucrose, but did not avoid fluids containing starch or Polycose. Therefore, rats possess a keen sensitivity for starch, a substance that is commonly thought to be tasteless. Furthermore, starch, Polycose and sucrose are discriminable from each other.