In toxicological studies, the test compound FOE 3440 A, a [(3,5‐dichloro‐2‐pyridyl)oxy]phenoxypropanoate with herbicidal properties, produced a severe increase in weight and an intensive induction of monooxygenases activity in the mouse, but not in the rat. Comparative subacute studies were performed with oral administration of 0, 5, 20 and, in some instances, 80 mg kg−1body weight to mice, rats, hamsters, dogs and rhesus monkeys. Liver enzyme activities were measured. The evaluationofthe enzyme activity results showed an unusually severe dose‐related induction of the monooxygenases [7‐ethoxycoumarin‐O‐deethylase (EOD), 7‐ethoxyresorufin‐O‐deethylase (EOR) and aldrin epoxidase (ALD)] in the mouse and a much weaker reaction in the other species tested.