The effects of several dietary additives that are related to stress levels on the heterophil to lymphocyte ratio (an indicator of stress) were studied in heat-stressed chickens from a White Leghorn population. There were six experiments 1) brewer's yeast at 0.5% in the diet; 2) ascorbic acid at 1000 ppm; 3) tocopherol at 250 ppm; 4) tryptophan at 0.5%; 5) niacin at 250 ppm; 6) lactic acid at 2%. Treatments were begun 3 days before measurements of heterophil to lymphocyte ratio. Two treatment groups were used in each experiment (heat stress versus heat stress + dietary additive). In Experiment 1 there was an additional group of hens without heat stress. Addition of yeast, ascorbic acid or tryptophan did not change significantly measurements for heterophil to lymphocyte ratio. Addition of tocopherol or niacin decreased significantly the heterophil to lymphocyte ratio of the heat-stressed hens (0.43 ± 0.07 vs. 0.65 ± 0.07 and 0.45 ± 0.07 vs. 0.66 ± 0.07, respectively; P < 0.05). Hens of the ascorbic acid, tocopherol, tryptophan and niacin groups had significant lymphophilia (P < 0.05). Heat-stressed hens with lactic acid added to the diet had significantly higher heterophil to lymphocyte ratio than hens without supplemental lactic acid (1.19 ± 0.20 vs. 0.62 ± 0.20; P < 0.05). The significant increase observed for the lactic acid group was due to a significant heterophilia (P < 0.05). The results suggest that tocopherol and niacin are effective to alleviate heat stress levels, whereas lactic acid reinforces the stress induced by the high temperature. © Verlag Eugen Ulmer GmbH & Co.