Since their beginnings in the late 1960's ion-selective electrodes have rapidly become one of the most important types of chemical sensor. They are commercially available for a large variety of ions, are widely used and have been characterised thoroughly by many investigators. Having attained this level of sophistication, research in this area today is often directed towards novel applications of such sensors. This thesis represents a study of certain clinical applications of sodium-selective electrodes. As ion-selective electrodes possess the ability to directly sense analytes, they are well suited to whole blood, plasma and serum critical analyte testing. These critical analytes are major electrolytes, blood gases and some metabolites. Analysers based on ion-selective electrodes can provide fast turn-around time for critical care testing. With this in mind, a flow injection analysis system has been developed for the determination of sodium in blood samples. Here, the advantages of flow injection analysis and potentiometric detection were combined with those of computer control, to produce a blood analyser which has the potential to be user friendly, analytically reliable, portable and applicable to continuous monitoring. Following this research, our attention turned to solid-state devices. Here, one opens up the possibility of mass fabrication of miniature, disposable, planar electrodes. Our efforts concentrated on the development of a potentiometric device, consisting of a solid-state sodium selective electrode and a solid-state chloride electrode. The device is based on a combination of screen printed Ag/AgCl electrodes and an ion-selective PVC membrane electrode. It was developed for use as a screening tool for Cystic Fibrosis, utilising the raised sodium level in sweat, which is a diagnostic indicator for the condition.