With the pregnant rat under ether anesthesia, rat fetuses were exposed on Day 17 of gestation to a taste/odor stimulus (mint) injected into the amniotic fluid and/or lithium chloride injected into the peritoneum. Behavior of injected fetuses was directly observed on Day 19 of gestation following chemomyelotomy and laparotomy of the female and immersion of the uterus into a warm saline bath. With these procedures, a series of four experiments was conducted to assess the behavioral effects of (a) the mint taste/odor alone, (b) the LiCl alone, (c) the pairing of mint and LiCl on the day of conditioning, and (d) the reexposure to mint after an earlier pairing of mint and LiCl. These experiments provide clear evidence that rat fetuses are capable of forming conditioned taste/odor aversions as early as Day 17 of gestation and, further, that rat fetuses are capable of expressing these learned aversions in utero.