A novel approach to estimating the current density required to directly activate axons involved in the reinforcing effect of brain stimulation is described. Self-stimulating rats received trains of cathodal pulse pairs via two adjacent, stimulating electrodes which were positioned in the lateral hypothalamus and oriented to lie in a plane transverse to the medial forebrain bundle. The first pulse of each pair was delivered through one electrode and, after varied delays, the second was applied to the other electrode in an attempt to detect a loss of stimulation effectiveness attributable to refractoriness of axons that penetrated the intersection of the two stimulation fields. In low-current tests, no change in the psychophysically scaled effectiveness was observed as the interval separating the pulses was varied but, at higher currents, the stimulation effectiveness rose when the delay between the pulses surpassed the refractory periods of these cells. Our inference was that greater currents produced progressively overlapping stimulation fields. Moreover, evidence of overlap was seen at lower currents in rats that had been prepared with smaller separations between the electrodes. Our estimate of the threshold current density for the most sensitive of the reward units is 1300 microA/mm2 when 0.1 ms pulses are used.