1. The effects of experimentally induced heat-stress on the embryonic development of bursa of Fabricius and thymus of the chicken were investigated by means of histological and enzyme histochemical methods. 2. In the experiments, 250 fertile eggs of the Ross 308 broiler strain were divided into two groups. The control eggs were maintained under optimal conditions (37.8°C and 65±2% relative humidity, RH) during the whole incubation period. Heat stressed eggs were maintained under normal conditions (37.8°C and 65±2% RH) until the 10th d of incubation and then exposed continuously (24 h per d) to high temperature (38.8°C and 65±2% RH). Blood and tissue samples were taken from 10 animals of each group at d 13, 15, 18 and 21 of incubation and at d 2, 4 and 7 post-hatch. Tissue samples were processed for enzyme histochemical methods in addition to routine histological techniques. 3. The results revealed that egg temperatures were higher than incubator air temperature. Long-term heat-stress (40.1-40.6°C egg temperature) retarded development of thymus and bursa of Fabricius. Peripheral blood ACP-ase and ANAE-positive lymphocyte levels of heat-stressed animals were lower than in the controls. 4. These results give some morphological evidence for immunosuppression induced by high temperature exposure during the embryonic development. Temperature distribution and air circulation in incubator should be questioned in the case of lower broiler flock immunity. © 2010 British Poultry Science Ltd.