1. Lysophospholipids caused the release of 45Ca2+ from isolated rat liver mitochondria incubated at 37°C in the presence of low concentrations of free Ca2+, ATP, Mg2+, and phosphate ions. The concentrations of lysophosphatidylethanolamine, lysophosphatidylcholine, lysophosphatidic acid and lysophosphatidylinositol which gave half-maximal effects were 5, 26, 40 and 56 μM, respectively. The effects of lysophosphatidylethanolamine were not associated with a significant impairment of the integrity of the mitochondria as monitored by measurement of membrane potential and the rate of respiration. 2. Lysophosphatidylethanolamine did not induce the release of Ca2+ from a microsomal fraction, or enhance Ca2+ inflow across the plasma membrane of intact cells, but did release Ca2+ from an homogenate prepared from isolated hepatocytes and incubated under the same conditions as isolated mitochondria. 3. The proportion of mitochondrial 45Ca2+ released by lysophosphatidylethanolamine was not markedly affected by altering the total amount of Ca2+ in the mitochondria, the concentrations of extramitochondrial Mg2+, by the addition of Ruthenium Red, or when oleoyl lysophosphatidylethanolamine was employed instead of the palmitoyl derivative. 4. The effects of 5μm-lysophosphatidylethanolamine were reversed by washing the mitochondria. 5. The possibility that lysophosphatidylethanolamine acts to release Ca2+ from mitochondria in intact hepatocytes following the binding of Ca2+-dependent hormones to the plasma membrane is briefly discussed.