Liquid junction potentials were determined in the pubic symphyses of guinea-pigs. The electrolytes studied were isotonic NaCl and its 1/10 dilution, and isotonic KCl. A theoretical treatment relates values found at various stages of relaxation to the density of immobile, negatively charged colloid of the symphysial connective tissue matrix. Spayed females were injected with small priming doses of oestradiol benzoate for 4 days, then treated with graded doses of relaxin, and potential readings made 8–10 hr. later. Linear relations were found between junction potentials and the logarithm of the relaxin dose. An electrometric unit of relaxin is defined as that amount of hormone producing a 50% decrement in the density of immobile colloidal charge at the symphysis. This approximates the standard 'palpation unit'. Animals treated with priming doses of oestrogen alone gave no significant potential fall, while large doses of relaxin alone showed low normal values. Thus the necessity for both hormones in evoking the full electrometric relaxation response confirms findings of previous authors on palpable relaxation.