Summary. Neutral lipids in the maternal uterine caruncular epithelium were studied by histochemical localization with Oil Red O. Results using a scoring system of 1 (negligible lipid) to 5 (maximal lipid) showed that intraepithelial lipid stores were minimal until Days 7–8 of the oestrous cycle and then increased to have a mean score of 4·4 on Days 14–15. In early pregnancy, although relatively high with a mean score of 3·2 at Day 15–16, such neutral lipids were significantly lower than those present at a comparable stage in the oestrous cycle. Thereafter, levels declined to a mean score of 1 on Days 21–23 of pregnancy. Such neutral lipid loss appears to be one of the first signs of the maternal response to the implanting embryo and precedes morphological evidence of transformation of either maternal or fetal tissues.