Thirty-two-week-old Hy-line brown commercial layers were used to investigate the additive effect of gamma-aminobutyric acid (GABA) derived from E. coli strains, on the productivity, egg quality, blood profile, cecal population and excreta noxious gas emission. Two hundred eighty-eight birds (46 ± 0.5 g) were fed 4 different levels of GABA (0, 25, 50 and 100 ppm), in a basal diet based on corn-soybean meal, for 5 wks. Daily egg production was expressed on a hen-day basis, and egg quality and blood profiles were determined using an egg multi tester and an automatic blood analyzer, respectively. The Lactobacillus and E. coli medium agar plates were incubated for 24 h at 37°C, under anaerobic and aerobic conditions, respectively. A Gastec gas sampling pump was used for noxious gas detection. Polynomial contrasts were conducted to measure the linear and quadratic effects for increasing GABA levels on all measurements. As a result of the current study, egg production (93.5 vs. 95.2, 95.4, 97.1), egg weight (62.2 vs. 64.3, 65.9, 66.8), and egg mass (58.2 vs. 61.2, 62.9, 64.9) during 32–36 wks showed significant improvement, as dietary GABA increased from 0 to 100 ppm (linear, P < 0.05). In addition, GABA supplementation was associated with increased eggshell breaking strength (4.28 vs. 4.48, 4.46, 4.44; quadratic, P < 0.05) and albumen height (8.16 vs. 8.58, 8.62, 8.78; linear, P < 0.05). Blood parameters, such as white blood cells, red blood cells, lymphocyte, cortisol, epinephrine and norepinephrine concentrations, were not influenced by GABA added into the diet; however, the haptoglobin concentration (15.0 vs. 12.3, 11.0, 10.8) was significantly decreased linearly, and IgG concentration (48.8 vs. 54.0, 54.3, 68.3) was increased quadratically in GABA-fed groups (P < 0.05). Lactobacillus populations (7.46 vs. 7.48, 7.58, 7.90) in the cecum were significantly increased, as dietary GABA supplementation increased (linear, P < 0.05). The ammonia (70 vs. 62, 59, 54) and hydrogen sulfide (12.0 vs. 9.7, 9.3, 7.7) gas emissions in the excreta were significantly decreased (linear, P < 0.05). These results suggest that diets containing GABA may beneficially affect productivity, egg quality, serum haptoglobin and IgG concentration, and excreta noxious gas emission. Besides, GABA may also improve the intestinal environment, by increasing the Lactobacillus population of layers.