Human and guinea-pig placental activity was measured in tissue extracts and in medium in which tissue fragments have been incubated. Lipase activity was estimated from measured rates of release of 3H-labelled fatty acids from a 3H-labelled triacylglycerol substrate. Whole tissue lipase activity was 1.6 +/- 0.3 units/g (mean +/- SEM, mumol of unesterified fatty acid release/min) in the human, and 87.1 +/- 7.3 units/g in guinea-pig placental tissue. 70% of the human and 56% of the guinea-pig placental activity was lost on treating the tissues with cold acetone. Sodium chloride (0.75 M) decreased activity by 50% in extracts from both species. Lipase activity was released into media when placental fragments were incubated. The addition of heparin increased the release of lipase from human placenta but not significantly from that of the guniea-pig. Insulin had no effect on placental lipase activity from either species. It is concluded that the placentas of both species contain at least two lipases. One of these, which constituted a large of the total, had the characteristics of lipoprotein lipase.