This research evaluated what ammonia concentration causes pullets or young laying hens to show an aversion. Because previous studies had shown conflicting data for laying hen aversion at 25 ppm ammonia, this aversion was tested with pullets and young laying hens in the current study. The test birds used in this study had not previously experienced elevated ammonia concentrations and thus their natural aversion could be determined. The evaluation of aversion was done using a newly developed Environmental Preference Test Chamber (EPTC). The birds were monitored to determine the percentage of occupation time (POT) under a higher ammonia concentration (25 ppm) vs. a low ammonia concentration (<10 ppm). The POT was measured over the total day (24 hr) as well as the lighted period only. Feed usage was also monitored as a possible indicator of aversion. Although none of the differences in POT or feed usage were significantly different, both lighted day POT and feed usage showed a trend of relationship (p= 0.13, p=0.16 respectively). This ongoing study aims to determine the birds’ aversion as part of a larger study to determine if ammonia or thermal discomfort drives preference.