The authors have found that a single dose of amitraz, a formamidine pesticide, produces death in mice which begins about two days after dosing and continues for up to 5 more days. To further examine the phenomenon, adult albino mice of both sexes were treated with either yohimbine (10 mg/kg, i.p.) or deionized water (6 ml/kg, i.p.), immediately before an injection of amitraz (600 mg/kg, i.p.) and twice daily thereafter for eight days. Male mice treated with water were more susceptible than water-treated females to the lethal effects of amitraz. In addition, yohimbine treatment significantly decreased the number of deaths in both sexes. These data suggest that an 2-adrenergic mechanism is involved in the delayed lethality produced by amitraz.