AbstractA double‐antibody ecdysone‐specific radioimmunoassay was used to clarify whether the effects on metamorphosis of the juvenile hormone analogue methoprene are correlated with changes in ecdysteroids level. It appears that a small ecdysteroids peak, 5 days before pupation, is responsible for the transition from inhibition to defective metamorphosis. Study of the changes in ecdysteroid titer in last‐instar larvae treated with the JHA 2 days prior to the appearance of the above small ecdysteroids' peak showed an immediate reduction in ecdysteroid level, followed by cyclic, successively reduced titer for about 20 days. After this period the larvae ceased to feed and entered to a diapauselike stage which ended in the death of the larvae. A similar effect on ecdysteroid titer and developmental arrest was exhibited by JHA‐treated first‐instar larvae. The mechanism of the interactions between JHA and ecdysteroid level deserves further inve