Thirty-seven pigs with an initial live weight of 60 kg were used to investigate the effects of daily exogenous porcine somatotropin (pST) administration at two dose levels (0 and 100 micrograms.kg-1.d-1) for a 31-d period on muscle fiber characteristics and meat tenderness of boars, gilts and barrows. Excipient boars and gilts had more alpha W and fewer alpha R fibers than did those receiving pST. The percentage of muscle fiber type for barrows was not affected by pST treatment. The administration of pST resulted in an increase in muscle fiber size for all three fiber types in all three sexes, but these changes were of greater magnitude in barrows (31.8%) and gilts (27.8%) than in boars (9.3%). Somatotropin negated the intrinsic sex effect differences in fiber area of the pigs. There was no difference in tenderness among excipient boars, barrows and gilts; however, with the inclusion of pST, shear force decreased in boars and gilts and increased in barrows. A high proportion of the pST-treated pigs contained giant fibers in the longissimus muscle. Furthermore, a small proportion of the pST-treated pigs exhibited pale, soft, exudative muscle. Whether the giant fiber anomalies occurred through increased muscle activity or from fibers undergoing degenerative changes was not determined.