The lipolytic activities with acid pH optimum were extracted from the acetone powders of the pregnant and lactating mouse mammary glands. Since the activities were strongly inhibited by human serum and albumin which were required for lipoprotein lipase assay, they were considered to be ascribable to lipase(s) other than lipoprotein lipase. Despite the fact that the amount of fatty acid released by the extract prepared from the glands in pregnancy (P-extract) was proportional to the incubation time, the release of the fatty acid by the extract from those in lactation (L-extract) ceased during incubation for a short time. The cessation, however, was avoided by preincubating the L-extract at 50 degrees C for 10 min before assay. In the presence of L-extract, the lipolytic activity in the P-extract was also almost completely lost after a short incubation. From these results, the existence of a heat labile inactivator of the lipolytic activities in lactating mouse mammary glands was suggested.