A photochemical equilibrium model of the high-latitude ionosphere has been developed. This model provides densities of the ionospheric constituents, N+2, O+2, O+, and NO+, from 85 km to approximately 220 km. These densities are then used to calculate Pedersen and Hall conductivities. A comparison of the model results with Arecibo and Chatanika radar observations was made, covering periods of solar minimum and solar maximum. The comparison showed the model to predict ionospheric densities to within 50% and conductivities to within 40% in the illuminated portion of the ionosphere. In regions of electron precipitation, the model showed good agreement with measurements. Results of this study indicate the following: (1) Ionospheric conductivity increases by a factor of ~1.6 from solar minimum to solar maximum conditions; (2) the portion of the ionosphere above 170 km can contribute as much as 40% during daylight and 80% during nighttime to the total height-integrated Pedersen conductivity; (3) the ratio of the height-integrated Hall to Pedersen conductivities is approximately 1.1--1.3 for sunlit conditions; this is appreciably lower than the value of 2 found in previous studies; (4) these and other factors indicate that, under certain conditions, the height-integrated Pedersen conductivity may be as much as 2--3 times larger than previously reported. ¿ American Geophysical Union 1988