We have obtained solutions of the coupled continuity, momentum, and energy equations for NO+, O2+, and O+ ions for conditions appropriate to the daytime high-latitude E and F regions. Owing to the rapid increase of the reaction O+ + N2 → NO+ + N with ion energy, high-latitude electric fields and consequent E⊥ × B drifts deplete O+ in favor of NO+. For electric field strengths less than about 10 mV m−1 the depletion of O+ is small, and the altitude profiles of ion density are similar to those found at mid-latitudes. However, for moderate electric field strengths (∼50 mV m−1), NO+ is substantially increased in relation to O+ and becomes an important ion throughout the F region. For these conditions the electron density has a tendency to become nearly constant with altitude in the range 160–360 km. For large electric fields (∼200 mV m−1), NO+ completely dominates the ion composition to at least 600 km, decreasing at high altitudes with a diffusive equilibrium scale height. Since the overall F region electron density decreases markedly with increasing electric field strength, it appears that high-latitude, daytime electron density troughs are directly related to the presence of ionospheric electric fields. In addition, since increases in the N2 density or the N2 vibrational temperature also affect ion composition and electron densities in a manner similar to that of electric fields, the observed daytime troughs may arise from both processes acting simultaneously.