A two-timescale chemical algorithm has been developed to solve photochemistry coupled with transport in the middle atmosphere. It is suggested that two continuity equations for each species be solved when transport processes prevent instantaneous chemical equilibrium. The simultaneous solutions of the two sets of equations correspond to quasi-equilibrium and transient or forced states of all the modeled species. The chemical solver is incorporated in a two-dimensional model to study the chemical-dynamical coupling in the upper stratosphere and the mesosphere for different timescales in a consistent manner. New Parameterizations for calculating photolysis rates in the Schumann-Runge bands and Schumann-Runge continuum are presented on the basis of an optimal k distribution method. Several distinct features of measured tracer distributions in the mesosphere can be simulated by the model. These include (1) the model daytime mean OH distribution with a secondary maximum in number density of ~6.5¿106 cm-3 around 70 km, (2) a semiannual oscillation in O3 mixing ratio around 85 km that characterizes the coupling effect between the OH-O3 photochemistry and O transport, and (3) diurnal variations of O3 in the mesosphere controlled by both fast varying local photochemistry and slowly varying HOx transported from below. There is no systematic underprediction of mesospheric O3 in our model comparison with the measurements. Our model also predicts the morphology of chemical heating rate around mesopause by exothermic reactions. From 80 to 95 km the dynamically controlled atomic oxygen distribution generates a latitudinal chemical heating rate that counters the radiative heating rate gradient. ¿ 1999 American Geophysical Union