This invention concerns a process for killing internal parasites, especially nematodes and cestodes affecting warm blooded animals such as sheep, cattle, swine, goats, dogs, cats, horses and humans as well as poultry by administering an effective amount of a compound of the Formula I: ##STR1## Certain of the compounds of Formula I are novel and in further embodiments of the invention provide novel compounds and compositions for use in the process of the invention. The compounds are readily prepared by conventional chemical reactions. Various pyridinyl acylhydrazones of Formula I demonstrate broad-spectrum anthelmintic activity in sheep upon oral and/or parenteral administration.