Contralateral and ipsilateral cortical somatosensory evoked potentials to selective activation of different diameter sural nerve fiber groups were recorded in cats. Activation of alpha fibers evoked a well defined positive-negative primary potential at the contralateral somatosensory area II. Activation of delta fibers in addition to alpha fibers resulted in another positive-negative wave of comparable amplitude with a latency approximately twice that of the early component. Isolation of the peripheral input to the delta fibers alone, by blocking the alpha fiber activity with the use of triangular pulses, caused the complete disappearance of the initial wave, leaving the second with no latency or amplitude changes. This demonstrated that the latter potential was evoked by delta fibers. Ipsilateral responses followed the same pattern, however with higher peripheral thresholds, longer latencies and smaller amplitudes. No cortical potentials could be recorded to activation of C fibers. The results of this study are at variance with previous observations about the cortical responses to small myelinated nerve fiber activity. The rationale of the use of triangular pulses for selectively activating nerve fiber groups is also discussed.