The data from accidental releases of toxic gas to the atmosphere are incomplete because of the lack of information concerning the measurement of the concentrations emitted. Their estimation based on plants' responses to toxic gas exposure would provide an interesting tool to improve feedback from the incidents. With this in mind, preliminary studies on the response of plants to several levels of gas exposures are required. A protocol for the evaluation of acute effects of ammonia on the terrestrial lens Lens culinaris (Medikus) has been determined. For the preparation of reproducible samples, a technique of pre-germination of the seeds and their transplantation after selection was adopted. The plants exposure has been carried in a hermetic enclosure with a cross flow of diluted ammonia. Various concentrations for various exposure times were tested. The visual observation of the plants made it possible to define the various behaviour patterns in terms of the ammonia concentration.