The mechanism of cationic amino acid transport in lactating mouse mammary gland was investigated. Two Na+-independent systems of arginine transport were discriminated on the basis of their sensitivity to leucine. The leucine-sensitive uptake of arginine (Km 0·4 mM) was through a broad specificity system that interacted with both cationic and neutral amino acids, and was inhibited by preloading mammary tissue with neutral amino acids. The leucine-insensitive uptake was identified as the y+ system (Km 0·76 mM). Preloading mammary tissue with cationic amino acids increased the uptake of arginine by the y+ system. Decreasing the pH of the external medium to 6·0 suppressed the y+ system-mediated uptake by ∼25%, whereas the broad specificity system remained unaffected. Lactogenic hormones upregulated the y+ system-mediated uptake of arginine in pregnant mouse mammary tissue cultured in vitro, although the broad specificity system remained unaffected. The y+ system-mediated uptake increased 2-fold with insulin alone and 4-fold with the combination of insulin, cortisol and prolactin.