Stress factors encountered in broiler chicken rearing include high temperatures, overcrowding, and diurnal range, which exert profound negative effects on the weight gain and productivity of broiler chickens. The potential of gamma aminobutyric acid (GABA), an excitatory neurotransmitter, combined with Saccharomyces cerevisiae fermentation products to reduce various stresses was evaluated. Each experiment was performed in a general environment, a high-temperature environment, an overcrowding environment, and an overcrowding•diurnalrange environment, and experimental groups were divided into (-) control group without stress, (+) control group with stress, and a GY50 group with GABA and yeast culture (50 ppm each) with stress. Weight gain, feed intake, and feed conversion ratio were measured, and stress reduction was evaluated through hematologic analysis.