Antagonism by physostigmine of the "running fit" caused by levorphanol, a morphine congener, in mice.

1. Drugs of the morphine type cause a stereotyped "running fit" in the mouse.2. The intensity and duration of this response are related to the dose.3. Measurement of this phenomenon serves as a good method for the quantitative comparison of drugs of this type and for the study of their antagonists.4. Intracerebral injection of physostigmine antagonized the "running fit" induced by a wide range of doses of levorphanol.5. The results are consistent with the hypothesis that drugs of the morphine type act by retarding the release of acetylcholine at some central cholinergic synapses.