Morphine and related narcotic agonists produce mixed depressant and/or stimulant effects in a wide variety of animals including man (1,2). These effects are not only species but also time and dose related. It is not gener ally appreciated that even a single dose of morphine in a given species, such as the rat, may have initial depressant and subsequent stimulant effects. We believe this is an extremely important concept and intend to provide evidence of its wide applicability in the pharmacology of narcotics .