The use of ion-selective electrodes, particularly the nitrate electrode, was explored for the analysis of ionic species in natural aquatic systems. Attempts to compensate quantitatively for the effects of interfering ions were unsuccessful. Ion-selective electrodes should not be used, therefore, when substantial interferences are present. Standard addition is generally the preferred technique with ion-selective electrodes. The cadmium electrode was used to determine the formation constant of the citrate complex of cadmium ion. Copper ion deficiency in algal cultures was studied and correlated with copper ion activity as measured by the copper electrode. The growth of the algae at minimum copper levels could be suppressed by the addition of chelating agent. (Author)