Phosphatidic (PtdOH) acid formation is recognized as an important step in numerous signaling pathways in both plants and mammals. To study the role of this lipid in signaling pathways, it is of major interest to be able to increase the amount of this lipid directly. Therefore, "caged" PtdOH was synthesized, which releases the biologically active PtdOH upon exposure to UV. Analysis of the product revealed that two 2-nitrophenylethyl (NPE) caging groups were coupled to the phosphate headgroup of PtdOH. To measure the quantum efficiency of uncaging, a fluorimetric assay, based on the notion that the NPE cage is an efficient quencher of pyrene fluorescence, was developed. Consequently, after NPE-caged PtdOH and (N-pyrene)-PtdEtn had been mixed in DOPC vesicles, the extent of photolysis of caged PtdOH can be quantified by monitoring the increase in pyrene fluorescence. Using this assay, a quantum yield of 9.6% was determined for the uncaging reaction. The swimming green alga Chlamydomonas moewusii deflagellates upon addition of PtdOH. This response was used to study the release of PtdOH in vivo. Algae incubated with caged PtdOH only arrested swimming after exposure to UV, indicative of PtdOH release. This effect was not observed in the absence of the caged compound or when a control caged compound (caged acetic acid) was added. Fluorescein diacetate staining was used to show that the cells remained viable after UV exposure. The anticipated effect of PtdOH release is confirmed by phase contrast images of UV-exposed algae showing excision of flagella. Together, these results show that caged PtdOH can be used to efficiently increase PtdOH levels, demonstrating that it is a promising precursor for studying PtdOH-dependent signaling.