Flavor aversions induced by thallium sulfate: Importance of route of administration. Neurobehavioral Toxicology and Teratology, 00: 00-00, 1985. Flavor aversions induced by intraperitoneal (i.p.) and oral (p.o.) administration of thallium sulfate were compared in a repeated trial, two-bottle preference test. Male Long-Evans rats (N=6/group) were given 30-m access to a 0.1% saccharin solution followed 20-m later by either i.p. or p.o. thallium sulfate (2.5, 5, 10 or 20 mg/kg), vehicle or nothing. Non-treated and vehicle-treated rats consistently preferred the saccharin solution, with relative saccharin intakes ranging from 0.65 to 0.85 over the three choice trials. On the first choice trial, flavor aversions produced by i.p.-administered thallium sulfate were marginal and occurred only at the highest dosage. In contrast, on the first choice trial, p.o.-administered thallium sulfate led to pronounced aversions at all but the lowest dosage. Saccharin preferences on the second and third choice trials resembled those obtained on the first choice trial. These results suggest that failure to obtain toxicant-induced flavor aversions may be due in part to the particular route by which the toxicant is administered.