AbstractThe effects of ammonium sulphate and sodium nitrate onPinus sylvestrisseedlings inoculated withPaxillus involutusandSuillus bovinuswere investigated. Weekly fertilization (26 weeks) positively affected the plant dry weight, but inhibited the mycorrhizal formation and decreased the root length/weight ratio. Ammonium treatments affected the seedlings more positively and the mycorrhizas more negatively than nitrate did. The effects on seedlings inoculated withP. involutuswere more pronounced than on those inoculated withS. bovinus. The possible mechanisms of the effects of nitrate and ammonium on mycorrhizas are discussed.