Several observations have suggested that the anthelmintic ivermectin can affect nematodes by non-oral entry into the nematode body. To investigate this possibility further, we refrigerated Caenorhabditis elegans at 5 C to prevent its locomotion and to block the pharyngeal pumping that is so prominent a feature of its feeding. Worms were exposed to ivermectin (1–25 µg/ml) at that temperature for 1 hr, after which the medium was replaced by unmedicated medium at room temperature. After 1 hr at room temperature the worms were examined and counted to determine the degree to which irreversible immobilization had occurred. The drug was significantly less effective at 5 C than at room temperature. This reduction in potency could be attributed to a general cold-induced decline in the rate of the biochemical processes involved in drug action. Alternatively, the reduction could be attributed to the cold-induced blockade of pharyngeal pumping, which would suggest that the efficacy of ivermectin is partially the result of oral intake of drug. The fact that antinematodal efficacy was not entirely abrogated and reached a significant level despite blockade of pharyngeal pumping supports the former interpretation and is in accord with earlier indications that ivermectin can enter by non-oral routes. This conclusion is further supported by the observation that ivermectin is active against the nonfeeding third-stage larva of Haemonchus contortus.