Background: The collagenase activity in the uterus in the process of post-partum involution has been studied. Using the biochemical investigation methods it has been determined that in the first three post-partum days the collagenase activity increases essentially and the intensity of collagenolysis becomes maximal. The results suggest that there is a close correlation between the decrease of the collagen level and the increase of the uterine collagenase activity. Material and methods: Experiments were performed on female rats with the weight of 16–200 gr. The total number of the animals included into the experiment was 141. Rats were divided into 9 groups: groups 106 – advanced pregnancy (17-19 days); group 7 – the 10th day after the delivery; group 8 – the 15th day after the delivery; group 9 – virgin females, control. The uterus was taken for investigation. Female, that gave birth to at least 6 newborns, were taken for the research. The uterus was preserved in the solution of 10% formaldehyde. Histological specimens were colored by hematoxiline and eozine by the procedure of Van Gieson and Mallory. Results: Electronically-histochemically the collagenase activity has been detected in lysosomes of macrophages and fibroblasts in the myometrium of the virgin uterus. In the post-partum period the collagenase is secreted into the intercellular space by smooth muscle cells, macrophages and fibroblasts to participate directly in the extracellular collagen resorption. Conclusions: The intense extracellular catabolism of the collagen in the postpartum period in uterus occurs with the participation of the collagenase of smooth muscle cells, while the intracellular collagen degradation is realized by the collagenase of macrophages and fibroblasts.