Palmitoyl-, myristoyl- and lauroylcarnitine destabilize small unilamellar vesicles of 1,2-dipalmitoyl-n-glycero-3-phosphorylcholine (DPPC) and 1,2-dimyristoyl-n-glycero-3-phosphorylcholine (DMPC) into multilamellar liposomes. Their effect on the bilayer is dependent on the acyl chain length of the acylcarnitine, the ratio of the lengths of the acyl chains of the phospholipid and the acylcarnitine and the molar ratio of the phospholipid to acylcarnitine but not the absolute concentration of the acylcarnitine in the solute. Sarcoplasmic reticulum vesicles are broken down by each of the acylcarnitines at concentrations below their critical micellar concentrations (CMC). These three acylcarnitines stimulate the Mg2+, Ca2+-ATPase activity in SR-vesicles to a certain maximum, after which a net inhibition is observed. The maximum degree of stimulation depends highly on acyl chain length: the shorter the chain length, the more effective. In the same concentration range where the Mg2+, Ca2+-ATPase activity is increased, the net Ca2+-uptake is markedly decreased.