SUMMARY Adult female rats undergoing their first lactation were experimentally weaned by removing their pups on day 4 of lactation. The macrophages of the mammary gland were labelled with trypan blue or horseradish peroxidase. Treatment with trypan blue showed the macrophages to be present in the connective tissue stroma from the 4th day of lactation to the 5th day of involution. However, on day 6 of involution they were congregated around the remaining islets of alveoli. Other cells which took up the dye were found in the epithelium of the alveoli. They were few in number on day 4 of lactation, increased numerically during the first 2 days of involution, and decreased again by day 6 of involution. Since they took up the dye in a similar manner to the lymphocytes of the regional lymph node, it is believed that they were lymphocytes. Treatment with ICI 46,474 in no way affected the course of mammary involution up to day 6. Treatment with horseradish peroxidase on day 4 of lactation or day 2 of involution resulted in labelling of the macrophages, which were found scattered in the stroma. However, by the 4th and 6th day of involution the macrophages were found to a greater extent in and around the remaining islets of alveoli.