Paralysis due to avermectin B1a and ivermectin of Angiostrongylus cantonensis was compared to that of phenylephrine (an alpha-adrenergic agonist) and strychnine (a cholinergic inhibitor). The paralyzing action of ivermectin (2.5 X 10(-9) g/ml) was inhibited by the single, simultaneous addition of picrotoxin (3 X 10(-5) M), whereas the effect of the drug (2.5 X 10(-7) g/ml) was reversed only when picrotoxin was given with cholinergic spasmogens such as pyrantel and eserine. Bicuculline (3 X 10(-5) M) had a similar antagonistic effect for picrotoxin, but bicuculline was less effective. The paralyzing action of avermectin B1a (3.6 X 10(-14) M, 3.0 X 10(-14) g/ml) was antagonized only when picrotoxin was given with cholinergic spasmogens such as pyrantel, eserine, and N-methylcytisine (N-MC), or alpha-adrenergic antagonists such as phentolamine and dibenamine. On the other hand, the paralyzing action of strychnine (3 X 10(-6) M) or phenylephrine (3 X 10(-5) M) was relatively uninfluenced by picrotoxin, but was antagonized by pyrantel and N-MC or dibenamine. These results suggest that a gabergic mechanism is involved in the paralyzing action of ivermectin, as well as avermectin B1a, in A. cantonensis.