A total number of 500 two-weeks old sexed chicks from each strain “Inshas and Matrouh" was used to study the effect of late heat stress on some physiological and immunological traits. The chicks were divided randomly into five equal groups “100 chicks for each group, 50 male and 50 female". The first group didn't receive any treatment and served as control, the second group was exposed to both early (at 4 weeks of age) and late (at 18 weeks of age) heat stress without anti-stress, the third group received the same treatment as second group with anti-stress, the fourth was exposed to late heat stress without anti-stress, the last group (fifth) was received the same treatment as the fourth group with anti-stress. The heat stress was for 4 hours / day for 5 consecutive days at 38-39 ºC inside the house and the anti-stress was vitamin C, where it was added at level of 3 gm (20% concentration)/liter drinking water from 1 day before heat exposure until the end of the treatment. The obtained results could be summarized as follows: Body temperature (ºC) in treated chicks was higher significantly (P≤ 0.05) than control and female chicks were higher significantly (P≤ 0.05) than males. Heat exposure increased respiration rate (RR) significantly (P≤ 0.05) in treated chicks than control and in "Inshas" females than males. In general, no significant differences were found between the two strains or between two sexes for respiration rate. Plasma total protein, albumin and globulin didn't affected by strain or sex or their interaction (strain x sex). Late heat exposure with vitamin C increased plasma total protein and albumin concentration significantly (P≤ 0.05) than other treatments. Globulin concentration had no trend with treatments. No differences were found between strains or sexes or treatments in glucose concentration. Treatment with heat stress (either with anti-stress or without anti-stress) had no effect on Ab´s production (humoral immune response) against SRBC´s. Also no significant differences were found between strains or sexes. With regard to Ab´s production against Newcastle disease (ND), no significant differences were found between strains or sexes. The treated groups were significantly lower in Ab´s production (P≤ 0.05) than control. The treatment with heat stress decreased significantly (P≤ 0.05) the humoral immune response against Newcastle disease (ND).