Studies were made of ethylene evolution by `Heinz 1350' tomato fruits from plants that had received nitrogen nutrition from ammonium or nitrate salts in soil-based media. Fruits of plants receiving ammonium nutrition had higher ammonium concentrations, higher occurrences of blossom-end rot, and higher rates of ethylene evolution than fruits from nitrate-grown plants. Fruits showing blossom-end rot had higher ammonium concentrations and higher rates of ethylene evolution than normal fruits only if the plants received ammonium nutrition. Ethylene evolution increased as fruits from nitrate-grown plants ripened but without a concurrent increase in ammonium concentrations in the fruits. Ammonium accumulation in fruits apparently induces blossom-end rot and enhances ethylene evolution, but ammonium accumulation does not appear to be a naturally occurring phenomenon in ripening fruits or in fruits that have blossom-end rot arising from other casual factors.