I am extremely honored and pleased to have the opportunity to write a few introductory words for this timely volume on Na + /It exchange. This is a field of investigation that I entered into by challenge and necessity, embraced with passion and fmally left in my quest for new discoveries in growth control. Ten years, one third of my scientific life, has been devoted to uncovering the mysteries of intracellular pH (PH;) regulation with respect to growth factor action. I got started on this new topic in 1980, when I heard a rather provocative hypothesis presented by Enrique Rozengurt at an ICN-UCLA Keystone meeting on "Cell Surface and Malignancy". He showed that all mitogens induced amiloride-sensitive Na + entry into resting cells and proposed that, if a compound stimulates Na + influx, it could be a mitogen. In support of his proposal Enrique reported that the amphipathic polypeptide, mellitin, which induced Na+ influx, was indeed mitogenic for 3T3 cells. This was only correlation at this stage. However, I was fascinated by this talk. I immediately approached Enrique to inform him of my skepticism about this beautiful story, and to indicate that I would only be convinced when I succeeded in isolating mutant fibroblasts lacking the amiloride-sensitive Na+ transporter. ''Good luck!" was his response.