ABSTRACT. Medeiros L.F.D., Rodrigues V.C., Vieira D.H., Souza S.L.G. de, Neto O.C., Figueiredo N. de, Pinto C.F.D., Miranda A.L. & Violento C.B. [Physiological reactions in goat breeds maintained under shade, sun and partially shaded areas.] Reações fisiológicas de cabras em diferentes ambientes e coeficiente de tolerância ao calor em cabritos. Revista Brasileira de Medicina Veterinária, 37(4):286-296, 2015. Departamento de Reprodução e Avaliação Animal, Instituto de Zootecnia, Universidade Federal Rural do Rio de Janeiro, BR 465 Km 7, Seropédica, RJ 23851-970, Brasil. E-mail: diasmedeiros@yahoo.com.br The experiment was carried out to measure the effects of thermal stress on the rectal temperature (RT) and respiratory frequency (RF), in animals of Boer and Saanen breeds, under the conditions of hot and humid climate of city of Rio de Janeiro, Baixada Fluminense, South East Region of Brazil; also the heat tolerance coefficient (HTC) of Amakiri e Funcho was applied on pure and crossbreeds kids. The goats were divided into three groups, each group consisting of four females from each breed group. Each group was subjected to different surroundings, constituted by three experimental treatment: treatment A, with a sun protected enclosed area; treatment B, a sun exposed area without covering; and treatment C, area with a 50% covered section and a 50% sun exposed area, which permitted free circulation of the goats. A Balanced Latin Square was used. The RT and RF of the goats, in the afternoon periods (l5h00), were higher, than in the morning periods (09h00). The animals kept in the sun presented much higher results, especially in the afternoon periods, than the animals in the other two confinement areas. There were no differences in the RT and RF of the groups maintained in the shade or in partially covered area. There were differences in the RT and RF measurements between the two breeds, in the morning periods and in the afternoon periods. Independently of treatment and daily (morning and afternoon) surrounding temperature variation, the average of RT and RF in the Boer goats were lower than the Saanen goats, which revealed in the present study to be more sensitive to thermic stress. By the application of Amakiri e Funcho coefficient of heat tolerance (CHT), the Saanen and Boer goats obtained CHT of 77.14 and 85.96, respectively. The cross breed, ¾ Saanen + ¼ Boer, ½ Saanen + ½ Boer and the three cross (½ Anglo-nubian + ¼ Boer + ½ Saanen), obtained 77.86, 81.96 and 83.80%, respectively, verified higher adaptation of the Boer and three cross.