The purposes of this study were twofold: to compare bupivacaine and tetracaine spinal anesthesia with regard to the zones of differential sensory blockade and to evaluate the time-courses of the widths of the zones of differential sensory blockade during spinal anesthesia. In 51 patients, the most rostral levels of sensory denervation to light touch, pinprick, and temperature discrimination were measured. There was no statistically significant difference in the height of sensory blockade in the 29 patients given bupivacaine and in the 22 patients given equipotent doses of tetracaine. The widths of the zones of differential blockade were also not statistically different between the two groups during onset, maintenance, and regression of anesthesia, except that the light touch-to-pinprick and light touch-to-temperature zones of differential blockade were greater with bupivacaine than with tetracaine 30 min after subarachnoid injection. The width of the zones of differential blockade also remained unchanged within each group during onset, maintenance, and regression of anesthesia. Changes in, and absolute levels of, blood pressure and heart rate were similar with both bupivacaine and tetracaine throughout. We conclude that zones of differential sensory blockade are essentially the same with tetracaine and bupivacaine, that the widths of the zones of differential sensory blockade remain constant during onset, maintenance, and offset of spinal anesthesia, and that bupivacaine and tetracaine are associated with similar changes in heart rate and blood pressure during spinal anesthesia.