In view of the possible association between clinical symptoms in patients with acute intermittent porphyria and monopyrroles and their oxidation products, a pharmacological investigation was undertaken utilizing mice and rats as the experimental animals to further evaluate this correlation. In mice 4-ethyl-3-hydroxy-3,5-dimethyl-delta4-pyrrolin-2-one was more toxic, and 4-ethyl-5-methoxy-3,5-dimethyl-delta3-pyrrolin-2-one less toxic than kryptopyrrole in the evaluation of behavioural, hypnotic and hypothermic effects. Experiments on the development of tolerance in rats to daily injections of kryptopyrrole have been performed and could serve as a model for the spontaneous recovery occurring in patients with acute porphyria. A new method for the determination of kryptopyrrole in urine is reported. It is based on radiolabelled aldehyde reacting with kryptopyrrole. This method differed from the regular Ehrlich aldehyde reaction, although the results of both methods depend upon dilution of the urine.