The intravenous toxicity to the rat of 36 pyrethroids has been examined. With two exceptions they cause either (1) T-syndrome, consisting of aggressive sparring, sensitivity to external stimuli, fine progressing to gross whole body tremor and prostration or (2) CS-syndrome, consisting of pawing and burrowing behaviour, salivation, coarse tremor, progressing to sinuous writhing (choreoathetosis) and clonic seizures. The two exceptions presented a TS-syndrome with salivation associated with the T-syndrome. No clearcut relationship between chemical structure and symptoms of poisoning has emerged through some generalisations are discussed.