Simple Summary In the arid south of Tunisia, goats are favored by herders due to their demonstrated resilience. In the present study, the effect of heat stress on the growth of Tunisian local kids from birth to weaning age was evaluated, with body weight being considered as an important economic trait in goat production. A significant negative impact of heat stress on goat performance, evidenced by reductions in body weight under the conditions of moderate and acute heat stress, was obtained. Such results are crucial for informing selective breeding strategies aimed at enhancing livestock resilience to environmental stressors like heat, particularly in arid regions where such conditions are prevalent.Abstract This study evaluates the impact of thermal load on the weights of Tunisian local kids using 24 models with cubic and quadratic Legendre polynomials, based on daily temperatures (Tmin, Tmax, and Tavg) on the day of weight recording and averaged over 7, 14, and 21 days before weighing. The deviance information criterion (DIC) consistently shows that cubic polynomial models offer a better fit than quadratic models, highlighting their superior accuracy in studying the effects of thermal load on kid weights. The models with the best fit utilized average or maximum temperatures over 14 or 21 days. The patterns of response were similar across the temperature variables and periods, showing a stable weight response at lower temperatures (thermoneutral region) followed by a decline as the temperatures increased. The weight loss was -125 g/degrees C beyond the moderate heat stress threshold (Tavg21 = 17.7 degrees C) and -450 g/degrees C beyond the severe heat stress threshold (Tavg21 = 25.3 degrees C) for Tavg21. The heat stress thresholds for moderate heat stress (HS1) were 8.6 degrees C for Tmin14, 27.4 degrees C for Tmax14, and 18.6 degrees C for Tavg14; moreover, for acute heat stress (HS2), they were 17.2 degrees C for Tmin14, 32.4 degrees C for Tmax14, and 25.5 degrees C for Tavg14. High variability in individual responses was observed, with differences in the slope of response ranging from 2.0 kg/degrees C for moderate heat stress to around 3.0 kg/degrees C for severe heat stress for Tavg. The correlations between the weights under different temperatures were low, indicating that rankings based on weight could change with varying heat conditions. The animals with larger weight levels generally demonstrated better heat tolerance, and those with good heat tolerance under moderate conditions were also likely to have good tolerance under severe conditions.