Triadimefon is a widely used systemic fungicide, yet there is little published information on its effects in mammals. The study describes the effects of triadimefon in male and female rats using a functional observational battery (FOB), motor activity (measured in a figure-eight maze), and operant performance (responding under a fixed-interval 3-min schedule). For the FOB, Long-Evans hooded rats were tested immediately before dosing and 0.5, 4, 24, and 48 hr after dosing with either vehicle, 30, 100, or 300 mg/kg triadimefon i.p. Prominent effects of triadimefon 100 and 300 mg/kg included increased arousal, stereotypes involving repetitive sniffing, head bobbing, and pacing, and self-mutilation. Dose-related handling-induced convulsions, changes in reflexes and sensory reactivity, hypothermia, and body weight loss were also significant findings. Doses of 30, 75, and 150 mg/kg triadimefon increased figure-eight maze activity whereas 300 mg/kg decreased activity. Thus triadimefon produced a neurotoxic syndrome which is unprecedented in pesticides and is similar in many aspects to that produced by CNS stimulants.