Previous studies indicated that the association of Pisolithus tinctorius with pitch pine (Pinus rigida) seedling roots prevented Al toxicity from developing when seedlings were exposed to 200-mu-M Al in sand culture. The ectomycorrhizal fungus reduced seedling Al uptake and maintained P nutrition when seedlings were cultured on one-quarter strength Johnson's solution containing 3.5 and 0.5 mM NO3- and NH4+, respectively. To broaden the scope of these findings, nonmycorrhizal and mycorrhizal seedlings were grown on one-quarter strength Johnson's solution modified to contain NO3-, NH4NO3, or NH4+. Seedlings were exposed to 0 or 200-mu-M Al for 6 weeks. Reductions in seedling height growth rate and final shoot weight in response to Al occurred in nonmycorrhizal seedlings grown with NO3-. Root weights were consistently reduced by Al under all N-source combinations. Increasing proportions of NO3- in the nutrient solution increased cation accumulation in roots and shoots and depressed tissue anion concentrations. The coprecipitation of Al and P(i) i roots of Al-treated seedlings further limited P availability in this treatment. Mycorrhizal infection maintained growth and foliar P levels under Al exposure, suggesting that Al-induced P limitation was a critical factor in nonmycorrhizal seedlings grown on primarily NO3- -based nutrient solutions.