During August and September 1986, 11 aircraft flights were made over the eastern Pacific Ocean and continental United States. The suite of observations included simultaneous measurements of peroxyacetyl nitrate (PAN) and active nitrogen (NOx=NO+NO2). At altitudes of 4.5--6.1 km in the middle free troposphere, PAN was usually 5--6 times NOx in maritime air masses and 2--4 times NOx in continental air masses. In air masses of tropical origin, or in the marine boundary layer, both PAN and NOx were typically less than 20--30 parts per trillion by volume, and the PAN to NOx ratio was less than one. The observations show that PAN can be a major component of the odd nitrogen budget in the middle free troposphere and strongly reinforce earlier views that the abundance is mainly governed by long-range transport processes including formation during transport and continental boundary layer to free tropospheric exchange of PAN and its precursors. Unlike reservoir HNO3, PAN can be transformed to active nitrogen and peroxy radicals by a variety of physical atmospheric processes that lead to air mass warming. Since NOx plays a critical role in determining photochemical O3 production, which in turn determines the oxidative power of the atmosphere, the observed large ratios of reservoir PAN to active NOx imply an important photochemical and dynamical role for PAN in the eastern Pacific remote free troposphere. ¿ American Geophysical Union 1990