This experiment aimed to test different combinations of temperature and duration of exposition to heat stress in young broilers, for a maximum efficiency of early thermal conditioning (TC) for application in practice. 162 male Cobb day-old chicks were submitted to one of three thermal conditioning treatments: 12 hour (T1) or 24 hours (T2) at 36-37,5°C at 3 days of age, or control (CTR) at a constant temperature of 28°C. Body temperature (BT) was considered as an indicator of TC success. It was recorded before and after acclimatization. Body weight (BW), food consumption (FC) and mortality (M) were recorded between 1 and 7 days. At day 5, there were significant differences (<.001) between the BT of treatments. Animals from T1 had the lowest BT (40.35°C), below T2 (40.71°C) and CTR (40.55°C). The productive parameters showed significant differences between for BW and FC after TC, but these differences were NS in older animals. Liver and spleen weights were affected by TC treatment, but not heart and bursa. The TC during 3 days at 12 h was considered as more suitable for transfer technology to producers. TC should be improved in order to maintain the effect until the final stage of the production cycle.