1. Extracellular responses to whisker deflections were recorded in the rat ventrobasal thalamic complex (VB) under urethan anesthesia. One-hundred eighty neurons were collected from 107 animals. 2. VB was sharply demarcated in sections reacted for cytochrome oxidase (CO) activity. Referring to 33 dye spots identified in coronal sections, the localization of 45 neurons could be mapped out. Whisker-sensitive thalamic neurons were confined to the ventral posteromedial nucleus (VPM). VPM points responding to caudal vibrissae were localized in the dorsal aspect of the nucleus, and rostral vibrissae were found to project to deeper points. Dorsal whisker rows tended to be represented by caudal VPM points while ventral whiskers were associated with more rostral points. 3. Of 134 neurons that were noted as either sustained or transient based on the response to a maintained whisker deflection, 50 were sustained-type neurons and 84 were transient-type neurons. The number of whiskers eliciting a response (receptive-field size) varied among neurons; from 1 to 5 (median = 1) among the sustained neurons and from 1 to 12 (median = 4) among the transient neurons. 4. A calibrated, controlled mechanical stimulator was used to deflect individual whiskers. The threshold velocity was measured at 10 mm from the follicle of the most effective whisker when it was deflected in the most preferred direction. Sustained cells (n = 33) had low threshold values, mostly less than 10 mm/s (58 degrees/s in terms of angular velocity) whereas transient cells (n = 62) displayed widely distributed values ranging from 1 to 100 mm/s (5.8-580 degrees/s). The median of velocity threshold values (and angular velocity threshold) was 3 mm/s (17.4 degrees/s) for the sustained type and 8.5 mm/s (50 degrees/s) for the transient type. 5. The response latency was determined using a supramaximal deflection. Sustained cells (n = 29) had slightly but significantly shorter latencies than transient cells (n = 64) (median, 7 vs. 8 ms). 6. Exponential ramp-and-hold deflection was applied to construct a tuning curve to determine the degree of contribution of amplitude and velocity components of a mechanical stimulus under threshold stimulus conditions. Sustained neurons (n = 13) were more dependent on amplitude than on velocity, whereas transient neurons (n = 20) were more dependent on velocity than on amplitude. 7. Fine rostral whiskers were represented exclusively by sustained-type neurons that had small receptive fields.(ABSTRACT TRUNCATED AT 400 WORDS)