It is generally accepted that lymphocytes associated with the mammary mucosal immune system of non-ruminants may be largely derived from gut-associated lymphoid tissue (GALT). The relationship between the mammary immune system and the GALT of ruminants has not been clearly defined. To address this question, we examined patterns of lymphocyte localization in sheep by 51Cr-labeled lymphocytes following infusion back into donor ewes. We found that lymphocytes taken from mammary lymph nodes of pubescent ewes returned preferentially to mammary nodes, while in prepartum and postpartum ewes, mammary node cells localized equally well in mammary and mesenteric lymph nodes. In contrast, ileal mesenteric lymph node cells from pubescent ewes localized equally well in mammary and mesenteric nodes, but in prepartum and postpartum ewes, localization in mammary nodes was markedly reduced. Comparison of the homing patterns of mammary, mesenteric, and peripheral lymph node cells indicated that mammary node cells behaved similarly to peripheral, rather than mesenteric node cells. This information may be relevant to the extent of communication between the gut and mammary gland in ruminants.