The objective of this study was to evaluate the effects of genotype x temperature interactions on hormonal heat stress indicators and performance traits. Two-hundred forty female one-day-old chicks were randomly assigned to a completely randomized design in a 5×2 factorial arrangement with 5 genotypes (Naked-neck, Na, from Ethiopia; New Hampshire, NH; Lohmann White, LW and F₁ crosses of Na with NH and LW) and 2 ambient temperatures (thermo-neutral, 18-20^oC; heat stress, 30-32^oC). Blood samples were taken from 12 randomly selected birds per genotype and ambient temperature at 22, 38, 51 and 65 weeks of age. Levels of corticosterone (CS) and 3,5,3′-triiodothyronine (T₃) were determined in blood plasma. Heat stress effects on egg production traits were most severe in LW and NH, least severe in Na and Na × LW, intermediate in Na × NH. Plasma T₃ level was significantly reduced by 27.9% in heat stressed genotypes. Plasma CS increased by 12.6% in heat exposed genotypes. However, inconsistent responses of CS and T₃ levels were observed at different ages. The Na and their F₁ crosses demonstrated relatively better thermo-tolerance than LW and NH hens. Levels of plasma T₃ hormone might be considered as indicator of long-term heat stress in hens.