Plant secondary compounds have an important role in defense responses against herbivores and pathogens. This study summarises published and some unpublished data from a series of fumigation experiments where Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies CL.) Karst.) seedlings were exposed to different concentrations of gaseous air pollutants, ozone (03), sulphur dioxide (SOz) or nitrogen dioxide (NOz), in growth chambers. Concentrations of monoterpenes, resin acids and total phenolics were studied. Overall, needle monoterpenes were not affected by pollutants. Only very high level of O 3 (600 ppb) decreased concentration of some individual monoterpenes in pine needles. 03 did not have effect on concentrations of resin acids in pine needles. In contrast, the concentration of some individual resin acids increased in Oa-exposed pine shoots and in Oa-exposed needles of one spruce done. The highest dose of SO 2 decreased concentrations of resin acids in pine needles, but low exposure levels did not have effects. However, SO s had no effects on the resin acids concentrations of spruce needles, except some minor individual compounds were affected in clonal spruces. Increased concentrations of resin acids was found in pine shoots exposed to NO 2. Total phenolics of needles were not affected by pollutants. These observations suggest that among secondary compounds there is variation in sensitivity to air pollutants and genetically different trees have different responses to air pollutants.