Abstract 31P-NMR has been used as a non-invasive probe to examine changes in pH and phosphate metabolism at fertilization of the sea urchin egg. A pH increase of 0.43 units was measured following fertilization. This value is in good agreement with measurements made with pH micro-electrodes and the DMO method, but is contrasted with the failure of other workers to measure a pH increase with 31P-NMR. Significantly the increase in arginine phosphate associated with fertilization was found not to be triggered by a pH increase induced by NH 4C1 activation. In addition we examined the 31P-NMR spectrum of sea urchin sperm and determined their intracellular pH.