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Heat stress is a major limitation to reproductive function of sheep because it has deleterious effect on estrus incidences, estrus intensity and embryo production, which finally affects their productivity. Adult Malpura ewes (14) with an average body weight of 30.8 kg were randomly allocated into 2 groups of 7 each. Animals of group 1 were maintained under shed throughout the study while animals of group 2 were exposed to thermal stress (45 degrees C/6 hid) for 8 weeks in the hot chamber from 10: 00 to 16: 00 h and then taken back to the shed. All the animals were subjected to 15-days of pre-experimental period for getting them acclimatized to feed and micro-environment. The ewes were synchronized for estrus by administering 2 injections of PGF(2)alpha (7.5 mg) at 10 days interval at their respective places. The onset of estrus, estrus duration and various sexual behaviours i.e. circling, tail fanning, head turning, standing and approaching to ram were recorded during the experiment. The ewes maintained under shed set in estrus earlier and had longer duration of estrus as compared to ewes exposed to thermal stress. The lambs born from the group 1 ewes had significantly higher birth weight than lambs from group 2 ewes. It is concluded from this study that exposure to heat stress treatment was sufficient to alter the onset of estrus, sexual behaviour, conception and lambing rates of ewes reared in the semi-arid tropical environment.