A histological examination was made of the udders of 39 goats inoculated into the teat canal of one side of the udder with heat-killed or living Str. agalactiae strain S 13 . The reaction to heat-killed organisms was a slight, patchy and transient migration of neutrophils into secreting acini. Histological findings in animals inoculated with living streptococci varied according to the type of clinical reaction (minimal, intermediate or marked as described previously). Minimal reactions were characterised by an early migration of neutrophils into the secreting acini, followed by a slight macrophage-fibroblast reaction in the interacinar tissue. These changes were regarded as non-specific. There was no evidence of penetration of streptococci through the duct walls. Marked reactions were characterised by an early migration of neutrophils into secreting acini. The essential feature in all marked reactions was penetration of streptococci through the duct and acinar epithelium to the lymphatic channels. This penetration was brief and the organisms were rapidly destroyed, but it initiated a marked macrophage-fibroblast reaction. There was extensive damage to secreting tissue and very early evidence of involution. The disease progressed rapidly and ended in involution together with a greater or lesser degree of fibrosis. Some of the larger ducts showed papilliform outgrowths and development of a stratified, pavement epithelial lining during the height of the disease. In some smaller ducts ingrowths of granulation tissue were found developing in some cases to a considerable size and involving surrounding lobular tissue in extensive fibrosis. Cases of intermediate reaction to inoculation showed the same changes as in the marked reaction cases, but with much less extensive involvement of udder tissue. Penetration of the epithelium of the duct system by the invading organism was essential to the development of the classical type of reaction that closely resembled bovine streptococcal mastitis. Its occurrence coincided in many cases with a rise in body temperature of the animal and with a high viable count of streptococci per ml. of milk. There was no essential difference between histological findings in goats infected experimentally and those reported in the literature in natural cases of bovine streptococcal mastitis.