1. Glass microelectrodes were used to record extracellular responses from 308 SI cortical neurons to deflections of the contralateral vibrissae in 21 unanesthetized, paralyzed rats. Controlled deflections of individual hairs were produced by means of an electromechanical stimulator. Fast green dye marks were made to aid histological reconstructions of electrode tracks. 2. TS) were observed throughout layers II--VI; "fast" cortical spikes (FS) were less frequently encountered and largely restricted to layer IV. Although both types of potentials had similar negative-positive waveforms. FSs were distinguished from RSs by their comparatively rapid time course, about half that of RSs. RS units (RSU) discharged spontaneously at rates of less than 1--15/s, whereas FS units (FSU) displayed rates of 15--50/s. The amplitudes of FSs, which were generally smaller than those of RSs, often decreased during high-frequency discharges. 3. With sinusoidal oscillations of a vibrissa FSUs responded more reliably and over a broader range of frequencies (3 to at least 40 Hz) than did RSUs , particularly in layer IV. In addition, FSUs typically responded to whisker deflections over a range of 360 degrees, whereas many RSUs in layer IV displayed sharp spatial-tuning charcteristics, responding over a restricted range of less than 90 degrees. 4. Of all units, 58% responded preferentially to stimulus transients (vibrissal movements), 32% displayed sustained responses to stimulus stead-states (fixed vibrissal displacement). For the remaining 10% of units the appropriate stimulus could not be specified; these units were particularly common in layer V. 5. Computation of quantitative stimulus-response relations showed that many units increased their rate of discharge with increasing stimulus intensities. 6. Most units were directionally selective, responding preferentially to deflections of a whisker in one or more of four quadrants. 7. In radial penetrations through the cortex there was a columnar patern so that units were activated at least by the same (i.e., the "principal") whisker. In a number of cases these could be directly correlated with the barrels. Of all units, 55% responded to deflections of single vibrissa only, the remaining 45% to 2--12 adjacently situated vibrissae. For most multiple-whiser units the responses elicited by deflection of any one hair activating the neuron was qualitatively similar to those elicited by deflection of any other hair activating it. The principal whisker of the penetration was typically associated with the most vigorous responses. 8. In layer IV, 85% of neurons responded to deflections of one hair only, the remaining 15% to two or more hairs. In layers II and III 39% of units were activated by more than one vibrissa; in layers V and VI multiple-whisker units predominated (64%). Whisker configurations in the deep layers were larger than those in other layers. 9...