Measurement of intracellular pH in sea urchin eggs by 31P NMR. The intracellular pH (pHi) of sea urchin eggs was measured using 31P nuclear magnetic resonance (NMR) spectroscopy. The mean value of pHi of unfertilized eggs was about 6.2 (H. pulcherrimus and A. crassispina) to 6.3 (P. depressus). In contrast to results of other studies, pHi of sea urchin eggs was not changed after fertilization. During exposure of the eggs to NH4Cl or procaine-containing natural sea water (NSW), however, pHi rose about 0.4-0.8 pH units; the pHi fell to its initial value upon washing the eggs with weak base-free NSW. These changes of pHi by weak base treatment agreed well with the data obtained by other workers. In order to understand the discrepancy of pHi changes in fertilized eggs between NMR data and other measuring procedures, we measured acid production and O2 uptake, so that CO2 accumulation and proton release did not result in alkalinization. the invariance of the fertilized eggs under anaerobic conditions; otherwise NMR showed a different answer from other measuring procedures, because of its particular characteristics such as non-destructivity and compartmentation of pH.