The Kathmandu Valley in Nepal experiences severe winter temperature inversions due to its location as a high altitude plateau situated in a semi-enclosed basin. This combined with a rapid increase in population and vehicle ownership has made air pollution of great concern in the valley within the last decade. Only a few sporadic studies have been done characterizing either the meteorology or the chemistry of the valley. This paper presents preliminary surface measurements of O$_{3}$ and NO$_{x}$ (NO + NO$_{2}$) from a field campaign in the Kathmandu Valley during January and February of 2003 and examines the photochemical processes affecting these trace gases. Peak ozone mole fractions ranged from 40 to 100 ppb, while peak NO$_{x}$ mole fractions were between 20 and 80 ppb. Diurnal variations of ozone, which are substantial, are compared to simultaneous NO$_{x}$ and micrometeorological measurements to infer factors influencing the production and loss rates of surface ozone and NO$_{x}$. A box model parameterizing the major processes thought to affect valley photochemistry and meteorology is used to further distinguish the relative effect of each precursor emission and model parameter on ozone mole fractions. The magnitude of the simulated maximum or minimum ozone mole fractions appears to be largely dependent on the deposition velocity and ozone levels aloft. The time at which simulated maximum or minimum ozone occurs seems to depend more on ultraviolet dissociation rates and the thickness of the surface and mixed layers.