Diurnal variations in concentrations of nonmethane organic compounds (NMOCs) were measured during August and September of 2000 near the top of the Williams Tower (elevation 253 m) in Houston, Texas. Ambient air was collected in SummaŽ passivated stainless steel canisters over five-minute intervals, every two hours between 0700 and 1900 local time (LT) and every three hours between 1900 and 0700 LT. The samples were analyzed by a cryogenic preconcentration/high-resolution gas chromatographic technique with flame ionization detection. Distributions of NMOCs at the tower will be compared with those measured in suspected source regions, and diurnal variations will be examined for evidence of photochemical processing of source emissions. Preliminary calculations of in situ oxidant production tendencies in Houston, Phoenix, and Nashville, made with a box photochemical model, will be presented to examine the sensitivity of the predicted values to the various hydrocarbon distributions of the urban centers.