Isoprene and its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) were measured over a 4 week period in July of 1995 at a rural/forest site near Nashville, Tennessee, as part of the 1995 Southern Oxidants Study (SOS) field intensive. High nighttime isoprene mixing ratios, measured during a 3 day period of stagnant high pressure, are reported. These high nighttime isoprene events are interpreted as a result of continuing emission of isoprene into a developing shallow nocturnal boundary layer in the early evening, followed by advective transport under the inversion to the measurement site. During some evenings, there is very rapid decay of isoprene just after sunset. These events occurred when the product 3>⋅2> was relatively large, consistent with loss via reaction with NO3. A chemical box model showed that isoprene decays were consistent with the NO3 mechanism but only for relatively high NOx conditions. This study indicates that nighttime processing of isoprene can be important for forested regions susceptible to high-NOx transport events. We also find that this nighttime NO3 chemistry can lead to conditions where, at least at the surface, a significant fraction of the NOy is in the form of organic nitrates that are products of the NO3-isoprene reaction and that the NO3-isoprene reaction can be the dominant NO3 sink. ¿ 1998 American Geophysical Union