Non-specific acid phosphatase activity was studied in the rat uterine luminal and glandular epithelium at oestrous, late dioestrous, and 6th d of pregnancy using the substrates beta-glycerophosphate and p-nitrophenyl phosphate, at both pH = 5.0 and pH = 6.0. At late dioestrous and 6th d of pregnancy, a significantly larger lysosomal population in the luminal and glandular epithelium was operating at pH = 5.0 rather than pH = 6.0. At oestrous, the epithelial lysosomes showed no preference for an operational pH = 5.0 or 6.0. Uniquely, the luminal epithelial lysosomes showed a significant preference for the hydrolysis of p-nitrophenyl phosphate over beta-glycerophosphate at 6th d of pregnancy, compared to no substrate preference at oestrous and late dioestrous. The activity of 5-Nucleotidase was also estimated semi-quantitatively at oestrous, late dioestrous, and 6th d of pregnancy in the luminal and glandular epithelium. 5-Nucleotidase activity was greatest in the luminal and glandular epithelium at 6th d of pregnancy. These findings are considered with respect to the possible mechanisms involved in implementing the changes and the relevance to blastocyst implantation.