Abstract.To study the effects of heat stress (HS) on the growth and reproductive performance of chicks, 1-day-old male Wenchang chicks were randomly selected and divided into control (CK) and HS groups. The two groups of birds were fed according to a routine. The chicks in the HS group were placed under HS for 2 h day −1 (temperature, 40 ± 0.5 • ; humidity, 63.0-80.0 %) until the sixth week. At the end of each week, six chicks were randomly selected from each group and dissected for pituitary and testicular tissues, which were then weighed and sectioned onto slides to observe the histological changes in pituitary and testis under a microscope. Our results indicated that compared with the CK group, with the increase in age, HS significantly reduced the feed conversion rate (FCR) and weight gain per week, and these changes were positively correlated. The pituitary and testicular weights and volumes of chicks in the HS group were significantly lower than those in the CK group (P < 0.05). For 3-week old chicks, the cross-sectional area of seminiferous tubule in chicks of the HS group was extremely significantly lower than that of the CK group (P < 0.01). Compared with the CK group, the seminiferous epithelium was thinner in the HS group, the arrangement of spermatogenic cells became loose and irregular, and the integrity of the histological structure of testicular tissues was also damaged. Therefore, the above results indicated that HS significantly impeded the growth and development of pituitary and testis in chicks.