Spontaneous and cutaneously driven unit activity was recorded in the hindfoot region of rat Sm1 neocortex under controlled intravenous infusion, at three selected rates, of the steroid anesthetic agent Althesin. Increasing depth of anesthesia decreased average spontaneous firing rates of 67 single units from 2.5 to 11 Hz during light anesthesia to 0 to 2.5 Hz in deep anesthesia. Thresholds to cutaneous stimulation for 58 units were innocuous (from 15 to 190 micron) using a 5-ms ramp displacement of the skin in the center receptive field. Responses from 13 sites on the hindfoot were classified according to response probability for each of the 58 units in reply to stimuli at 1.5 times center receptive field threshold. Small receptive fields were seen only under conditions of deep anesthesia, considerable expansion of both center and excitatory surrounds occurring with lighter anesthesia. Mean values for total receptive field size (center plus surround excitatory receptive field) were 10.8, 7.8, and 3.4 sites, respectively, for light, moderate, and deep anesthesia. The size of the receptive field was also influenced by stimulus repetition rate; moderate increases of this and anesthetic depth could eliminate substantial receptive fields. Surround inhibition of evoked activity was more effective in deeper anesthesia with little effect in light anesthesia. We suggest that receptive field expansion in light anesthesia arises from a relative increase in excitability of afferent pathways and an accompanying increase in the preponderance of surround excitation vis à vis surround inhibition.