Tomato plants ‘Campbell 1327’ grown in peat with NH4 nutrition had no visible symptoms of NH4 toxicity, while severe symptoms of NH4 toxicity were displayed in solution or sand culture. Growth of plants was much better with NO3-N than NH4-N in sand, vermiculite, or solution culture; but in peat, growth of NH4-treated plants equalled or exceeded that of NO3-treated plants in sand, vermiculite, and peat. The total dry weight of NH4-treated plants grown in peat was 2, 2.5, and 3.4 times higher than plants grown in vermiculite, sand, and solution culture, respectively. Content of uncomplexed ASHS.109.3.406 in NH4-treated plants grown in peat was reduced markedly compared with other media. NH4-treated plants grown in sand, vermiculite, and solution culture, displaying NH4 toxicity symptoms, had a total amino acid:free NH4+ molar ratio < 2, compared to 6-8 with NO3. For NH4-treated plants grown in peat the ratio was similar to that of NO3-treated plants. Asparagine and glutamine in NH4-treated plants grown in peat were 3.5 and 11.3 times higher than with NO3-N, indicating a high efficiency in detoxification of NH4 through incorporation into these amino acids. The medium on which a plant is grown can have a marked influence on the plant response to N form.