Ecdysteroid and juvenile hormone titers were determined in the whole body of females of Aedes detritus and A. caspius. Since both hormones were assayed from the same extract, this method allowed determination of their simultaneous variations during egg formation, i.e., from the time the females emerged until the onset of oviposition. A drastic hormonal increase was observed at the beginning of vitellogenesis. This increase occurred as two high and sharp peaks, the first of ecdysteroids and the second, which took place 8 hr later, of juvenile hormones. The two peaks together lasted less than 12 hr, with the highest level at about 3 X 10(-7) mumol/mg fresh tissue. After the juvenile hormone peak, the oocytes entered into stage III/b, the time at which the intensive phase of vitellin accumulation in the eggs begins.