We present ground-based and aircraft measurements of photochemically relevant trace gases from the southern part of Switzerland from summer 1994. The region is adjacent to the Po Valley and exhibits the highest ozone concentrations within Switzerland. O3 concentrations of up to 166 parts per billion by volume (ppbv) were measured. Isoprene was 3 ppbv on the ground and 0.3 ppbv at 1000 m altitude on average in the afternoon and dominates the volatile organic compound (VOC) reactivity toward OH at the ground. Measured HONO concentrations of 0.2 ppbv in the afternoon are much higher, as can be explained from gas phase reactions alone. Radical concentrations are derived from steady state calculations using ground-based measurements in the afternoon. The resulting concentrations for OH, HO2, and RO2 are 3.1¿106, 1.1¿109, and 1.3¿109 molecules cm-3, and NO have the largest influence on the estimated OH concentration, followed by O3, photolysis frequency of O3, and HONO. HO2 and RO2 concentrations are most sensitive to changes in HONO, O3, and photolysis frequency of O3. The estimated radical production is larger than the NOx emissions, suggesting low-NOx chemistry. We calculated indicators for NOx or VOC sensitivity and show that reduction of NOx emissions would be more efficient in reducing O3 than VOC reduction upwind of the measurement site.¿ 1997 American Geophysical Union