An open-field activity test was developed for studying the effect of a single oral dose (range of 0.1-5 mg/kg) of L-deprenyl on spontaneous behavior in the dog. A computer program was used to quantify observations of locomotor activity, directed sniffing, urination, grooming, inactivity, jumping, rearing, and vocalization during a 10-min baseline and posttreatment session. Three dose-dependent behavioral changes were observed: an overall decrease in directed sniffing, an increase in total locomotor activity in females, and a decrease in frequency of urination in males. These effects were only seen at the dose levels of 2 mg/kg or higher. Computer-assisted tracings of behavioral patterns showed increased stereotypical behavior and decreased exploratory behavior at the high-dose levels. These behavioral effects are most likely due to either increased levels of phenylethylamine resulting from inhibition of monoamine oxidase B and/or the production of amphetamines as a result of the metabolism of L-deprenyl.