The terrestrial snail Cepaea nemoralis, when placed on a 40 degrees C hot plate, lifts the anterior portion of its foot. The latency of this response is influenced by morphine and by naloxone in a dose-dependent and time-dependent manner. Morphine increases the time taken to respond, whereas naloxone reduces it. Furthermore, naloxone abolishes the effect of morphine. These results indicate that an opiate system may have a role in this behavior, which resembles that reported in vertebrates.