This study describes a new parameter for quantifying the post-caloric nystagmus, the active duration, or the total periods where appropriate nystagmic response existed due to caloric stimulation. On a pathological population of 211 cases, the new parameter records a diagnostic sensitivity of 56% for the canal paresis and 59% for the directional preponderance, improving the sensitivity levels corresponding to the duration of the response according to classical criterion. Being easy to calculate, the active duration parameter is considered to be appropriate for the quantity valoration of the post-caloric nystagmus by means of computerized technics.