A small, flexible catheter-tip pH electrode has been developed for continuous measurement of intravascular pH in man and animals. The electrode utilises a pH sensitive polymer membrane rather than pH glass. The electrode passes through the lumen of a conventional cardiac catheter, by which it may be introduced to any desired site in the circulation. The electrode has no cross-sensitivity to other ions, to osmolality or oxygen tension. It is insensitive to flow and pressure. The response time is less than 300 ms, and the mean driftin vivois 0.01 pH units/h. Continuous recordings of intravascular pH in animals have been made for up to Th. The electrodes can be sterilised, and have been used in man.