Alterations in the skin temperature of the trunk during spinal anesthesia, using either tetracaine, 6 mg, or lidocaine, 50 mg, were monitored at intervals by means of a General Electric Spectrotherm 2000® Thermographic Imager and recorded on Polaroid® 107C photographic film. The upper level of sensory blockade was determined with each thermograph by recording the most cephalad dermatome at which analgesia to pinprick occurred. The upper limit of diminished sympathetic activity was assumed to be the most cephalad dermatome at which skin temperature elevation occurred. In all cases, the uppermost level of temperature elevation was cephalad to the upper limit of sensory blockade. Assuming that temperature elevation reflects diminished sympathetic activity, the mean sympathetic-sensory differential for lidocaine, 50 mg, was 6.00 (±SE 0.70) segments, and for tetracaine, 6 mg, was 6.70 (±SE 0.50) segments. Arrival of the temperature elevation “front” at the fourth thoracic dermatome and above was associated with decreases in mean arterial pressure.