In rats infected with different worm burdens of Angiostrongylus cantonensis, the loss of body weight was proportional to the increasing dose of infection. Body weight was correlated inversely with female worm burden (r = -0.669), the sum of mean numbers of larvae per gram of faeces (LPG) (r = -0.527) and lung weight (r = -0.761). Lung weight was positively correlated with female worm burden (r = 0.785) and the sum of mean numbers of LPG (r = 0.685). On the other hand, in rats infected with larvae developed from gamma-irradiated first-stage larvae (Experiment B) or infected rats treated with the drug ivermectin (Experiment C), body weight was inversely correlated with the sum of mean numbers of LPG (r = -0.798, Experiment B; r = -0.670, Experiment C) and lung weight (r = -0.862, Experiment B; r = -0.812, Experiment C), but not with female worm burden. Furthermore, lung weight was positively correlated with the sum of mean numbers of LPG (r = -0.783, Experiment B; r = 0.899, Experiment C), but not with the female worm burden. From these results, it is concluded that in the later pulmonary phase of infection in rats the pathogenic effects may be closely related to the increasing number of first-stage larvae produced by female worms.