The primary action of the anticoccidial drugs methyl benzoquate, buquinolate, and meticlorpindol is to prevent the growth of, rather than to kill, coccidia. In the case of Eimeria tenella in the chick, only the sporozoite when it has first invaded the host cell is susceptible to this action, but this is due not to lack of susceptibility of later stages, but rather to failure of the drug to penetrate deep into the gut wall. Later stages of E. brunetti are susceptible to methyl benzoquate given in the food as are later stages of E. tenella if the drug is given intravenously or the experiments are carried out in the chick embyro. Methyl benzoquate given intravenously in the chick forms a depot, mainly in the lung, which is able to protect against lethal coccidial challenges for several weeks. No success was achieved in explaining the mode of action of the drugs at a molecular level, although the observations would be consistent with an inhibition of nucleic acid synthesis.