A zonally-averaged general circulation model is presented, and some experiments with it described. The model is based on the primitive equations of motion, except that the zonally averaged zonal wind is constrained to be in a "spherical geostrophic equilibrium" with the geopotential. The effects of the large-scale eddies on the zonally averaged flow are parameterized using a scheme based on the work of Green (1970). The solar and infrared radiation schemes allow interaction with the cloud cover predicted via a hydrology cycle. When integrated, the model produces realistic energetics and fields of momentum, temperature and relative humidity.The effects of moisture on the model circulation are examined. Its inclusion intensifies and narrows the upward branch of the Hadley cell, and generally reduces the model ZAPE. The response of the surface temperature to changes in the solar constant is reduced by moisture, especially in low latitudes. Relative humidity is found to have an inverse relationship with surface temperature, if surface albedo is fixed. However, the presence of an ice sheet reduces low level relative humidity. Implications for simple energy-balance models and climate sensitivity are discussed.