We investigated behavior, physiology, sensitivity to exogenous application of ecdysone, and nerve terminal structure for differences between the reduced ecdysone genotype, ecd1/ecd1, and wild-type control ecd1/TM6B animals during the early and late third instars when raised at 25°C. The ecd1 mutants were able to survive through larval development and form pupae. However, the results demonstrate that the time to pupation is lengthened by about 50 h for theecd 1 /ecd 1 as compared with the wild-type control siblings. In addition to the lengthened larval cycle in the mutant, ecd1/ecd1 animals, they also display behavioral differences as compared with controls. The rate of body wall contraction and mouth hook movements are reduced in the early third instar of ecd1/ecd1 as compared with controls. The physiological measure of excitatory junction potential amplitude for the combined Is and Ib terminals did not reveal any differences among the two genotypes during the early third instar but the synaptic strength is reduced in the late third instars for controls. Application of exogenous ecdysone is still effective during the late third instar for the ecd1/ecd1 but not the controls. This suggests that endogenous production of ecdysone have already taken place in the wild-type but not the ecd1/ecd1 larvae, thus the rapid nongenomic responses could still be observed in the late third ecd1/ecd1 larvae. Structurally the number of varicosities and the terminal length showed significant differences between ecd1/ecd1 and the wild-type ecd1/TM6B genotype in the late third instars.