The central distribution of the terminations of primary afferent fibers from the gastrocnemius-soleus muscle in the cat was examined with the method of transganglionic transport of horseradish peroxidase. At the segmental level, the main projection areas were found to be laminae I and V-VII; in the rostrocaudal direction, the terminations extended from the third sacral segment to nucleus Z. A 40% solution of horseradish peroxidase in 2% dimethylsulfoxide was applied to the central cut ends of the muscle nerves in an open pool for several hours and subsequently was removed. A capsule of tracer solution applied during the survival period of the animals was found to result in additional labeling due to peripheral leakage. The tissue sections were processed with tetramethylbenzidine. Termination fields were consistently observed ipsilaterally in: lamina I from the L4 through S3 segments, being most dense in L6 and S1; lateral lamina V in L6 and S1-3; medial laminae VI-VII from L5 through S3; medial Clarke's column from L1 through L4; the ventral aspect of the gracile nucleus; and, nucleus Z. Little or no labeling was found in laminae II-IV in experiments in which peripheral leakage of tracer solution was prevented. The distribution of reaction product in laminae VI-VII and Clarke's column corresponds to the projections of large-diameter afferent fibers from the gastrocnemius-soleus muscle. The projections to laminae I and V, which are attributed to small-diameter sensory fibers, indicate involvement of these laminae in sensory modalities mediated by slowly conducting muscle afferent fibers, e.g. deep nociception. This pattern contrasts strongly with the central projections of cutaneous fibers, which terminate heavily in laminae II-IV, but resembles the central distribution of fibers from tooth pulp and viscera.