An apparatus is described which negates the influence of rats' position preferences by presenting alternative solutions at the same location. The licks for both solutions were monitored over consecutive short intervals by lick detectors with computer capture of data. Rats given a choice between water and dilute solutions of the high-potency opiates etonitazene (1.0-5.0 micrograms/ml) or fentanyl (10-50 microgram/ml) either licked equally for the two solutions, or gradually developed a preference or aversion regarding the opiate over the course of several days. In contrast, preferential licking for solutions with a definite taste, saccharin or quinine, was established in hours. These data indicate that the taste per se of these opiates is not aversive to rats, and that preferences for or aversions to the opiates have some other base, presumably one or more actions on the central nervous system.