The aim of present study was to examine characteristics of milk yield and secretion of growth hormone (GH) and insulin in Japanese Black cows (beef type) during lactation. (Experiment 1) Milk yield and plasma concentrations of hormones in Japanese Black cows were compared with those in Holstein cows (dairy type). Japanese Black cows had smaller milk yield and secreted less GH but more insulin than Holstein cows. (Experiment 2) Effects of GH-releasing hormone (GHRH) on milk yield and plasma concentrations of hormones in Japanese Black cows were examined. Treatment with GHRH induced increases in milk yield (17.4%) and in GH and insulin concentrations.