Plasma testosterone levels of reindeer increased from 1 ng/cm3 in August to 30–60 ng/cm3 in mid-September. By late October, when rutting activity had almost ceased, the testosterone concentration had declined to barely detectable levels. The seasonal pattern exhibited by caribou was similar except the peak levels associated with rutting occurred about 1 month later than reindeer. The seasonal changes in plasma testosterone levels of two hybrid calves were studied. Peak testosterone levels of 5 and 3.8 ng/cm3 were attained in late October.The influence of the endocrine glands on antler renewal and casting, and the rhythmic growth pattern exhibited by caribou and reindeer are discussed.