During the STRATOZ III flights in June 1984, approximately 160 measurements of peroxyacetylnitrate (PAN) were made with a specially designed gas chromatograph on board the Caravelle 116 airplane. The measurements cover a latitude range from 70¿N to 60¿S and altitudes up to 12 km. The results show that PAN is present throughout the troposphere at low but measurable levels. Southern hemispheric data from clean marine air masses show levels of 7--10 ppt, with little variability up to about 10 km altitude. In the southern uppermost troposphere and lowest stratosphere a significant increase in PAN can be seen, indicating substantial in situ formation of PAN at these altitudes. The situation at lower latitudes in the northern hemisphere is similar as long as the air masses are free of anthropogenic influence. At mid and high northern latitudes, considerably higher PAN mixing ratios (sometimes more than 100 ppt) are found even outside continental areas. These data also show a rather high variability. This can be ascribed to the influence of North America and Europe as source areas for PAN and PAN precursors. ¿American Geophysical Union 1987