Ambient concentrations of key trace gases were measured concurrently at a rural site in the eastern United States during the summer of 1986. The extensive data set allows the evaluation of certain aspects of photochemical oxidant models. A 4-day high-pressure period is chosen for a comparison of the model predictions with observations. The emphasis of this analysis is on the discussion of the photochemistry of the reactive nitrogen species and their role in shaping the ozone budget. Observations of the NO2/NO ratio provide an estimate of the peroxy radical concentration at the site. The peroxy radical concentration so derived indicates a significant in-situ O3 formation which is in accordance with the observed diurnal variation of O3. Isoprene, a natural hydrocarbon, is shown to be much more important than the anthropogenic hydrocarbons in this O3 formation. At this rural site NO, NO2, HNO3, and peroxyacetyl nitrate (PAN) account for 85% or more of NOy (the sum of the reactive nitrogen species). The diurnal cycle of the reactive nitrogen species and their partitioning is determined by transport and photochemical conversion. At night in the boundry layer defined by a low-level inversion, NOx is the major NOy species at this site. Nitric acid and PAN are suppressed by dry deposition and, in the case of PAN, thermal decomposition. During the daytime the NOx contribution to NOy decreases first as higher concentrations of HNO3 and PAN are mixed downward upon the breakup of the nocturnal inversion followed by NOx conversion to other NOy species in photochemical reactions. ¿1991 American Geophysical Union