ABSTRACTThis study was concerned with the relationship between stimulus intensity (concentration of citric acid) and parotid salivary response in man as reflected in four measures. Although this relationship varied with the measure employed, it was in no case linear. It was concluded that optimum concentration of citric acid for stimulating parotid salivary secretion in man was 1.0% and that the measure which differentiated best between the weakest acid stimulus and water as a control and between the two lowest concentrations of acid was the average quantity of evoked saliva per trial.