The theory that β-ecdysone initiates developmental changes during insect metamorphosis bycausing an increase in intranuclear levels of potassium, together with a concomitant decreasein sodium levels, has been investigated by two methods. First, imaginal discs from late thirdinstar larvae have been cultured with 0·2 μ/ml of β-ecdysone together with inhibitors ofactive ion transport. Non-specific inhibitors, which may have general effects on sulphydrylgroups, such as iodoacetic acid, iV-ethylmaleimide, ethacrinic acid and furosemide, inhibitboth eversion and differentiation at concentrations of from 10−3m to 2 × 10−3m. Ouabain, theonly specific inhibitor of the active transport of Na+ and K+ across membranes, had noeffect on development even at a concentration of 10−2m. Second, a medium containing raisedlevels of K+, and reduced concentrations of Na+, neither initiated disc development in theabsence of β-ecdysone, nor stimulated development induced by suboptimal levels (0·02 μ/ml) of β-ecdysone, either in the presence or absence of ouabain. These results suggest that β-ecdysone induced morphogenesis is not dependent upon Na+ and K+ concentrations, or onthe activity of an ouabain-sensitive ion pump.