Cathepsin L has been proposed to be involved with the endothelial-chorial type of placentation in the cat. Little information concerning the presence and secretion of cathepsin L is available for a species with noninvasive epitheliochorial placentation such as the pig. Cathepsin L activity in uterine flushings and endometrium from gilts during different days of the oestrous cycle and early pregnancy was analysed through specific substrate metabolism and Western blot analyses with antiserum against cat endometrial cathepsin L. This antiserum was utilized to determine the cellular localization of the enzyme within porcine endometrium. Cathepsin L activity within uterine flushings was elevated on Day 15 of the oestrous cycle and early pregnancy, with activity declining on Day 18. Cat cathepsin L antiserum cross-reacted with a group of 46, 40 and 38 kDa uterine proteins and detected a product within the surface and glandular epithelium of the endometrium. The appearance of the 40 kDa protein was first detected on Day 10 of the oestrous cycle with the 38 kDa proteins appearing on Day 15 and 18 of pregnancy. The 40 and 38 kDa uterine proteins appear to be steroid regulated as 12 days of progesterone administration is necessary to detect the proteins and cathepsin L activity.