1. Addition of arachidonic acid (AA) to Ascidia ceratodes oocyte homogenates results in its rapid oxidation to several polar products. 2. AA oxidation in homogenates has both calcium independent and calcium stimulated components. 3. Calcium or AA addition to an oocyte homogenate stimulates O2-consumption. 4. Stimulation of homogenate O2-consumption by AA and calcium is additive. 5. Intact eggs oxidize AA to products similar to those detected in vitro. 6. Quantitatively total AA oxidation was similar for unfertilized and fertilizing eggs and dividing embryos, while qualitative differences were detected for the three stages. 7. These results demonstrate the presence of lipoxygenase-like, peroxidizing activity, in Ascidia eggs that is capable of producing products potentially important to the control of early metabolic events during development.