Porcine muscle has a unique grouped arrangement with 3-70 Type I fibers occurring in clusters surrounded by Type II fibers. The deep medial portion of the porcine semitendinosus exhibits a regular pattern of extensive Type I grouping whereas the superficial or lateral portion of the muscle exhibits Type II predominance. Mean values for terminal innervation ratios were 1.00 +/- 0.01 and 1.02 +/- 0.01 in normal superficial and deep semitendinosus respectively. Subterminal axons, therefore, do not branch intramuscularly and innervate only one muscle fiber in normal porcine muscle. Following nerve crush and subsequent reinnervation, fiber type conversion occurred which resulted in a fiber type grouping pattern dissimilar to the normal grouped arrangement. Significantly (P less than 0.01) elevated terminal innervation ratios (1.45-2.15) were measured in reinnervated muscle as a result of extensive branching of subterminal axons, but the percentage of Type I and Type II muscle fibers was unchanged. It was concluded that: (1) nerve crush causes distal nerve degeneration, loss of the normal fiber type pattern, and extensive collateral ramification of subterminal axons; (2) collateral reinnervation imposes a neuronal influence on muscle fibers which dictates transformation of all muscle fibers innervated by a single subterminal axon to a uniform histochemical profile; and (3) the type grouping observed in normal porcine muscle is not a result of neuronal influence mediated by collateral branching of subterminal axons.