Twenty crossbred beef steers (initial BW 182 +/- 1.8 kg) were used in a 2 x 2 factorial to determine the effects of implantation with Synovex-S(R) (SYN; 20 mg 17-beta estradiol benzoate + 200 mg progesterone, ear implant) and administration of recombinant bovine growth hormone (Somavubove(R) SbV,.1 mg . kgBW(-1). d(-1), i.m.) on the morphology of six muscles (longissimus, psoas major, supraspinatus, triceps brachii, semitendinosus, rectus femoris) of growing steers. Implantation with SYN decreased the percentage distribution of FOG fibers and increased FG fibers in the supraspinatus and rectus femoris muscles (P <.05). Steers treated with SYN had a larger area of SO and FG fibers in the psoas major muscle (P <.05). The administration of SbV decreased the percentage distribution of FOG fibers and increased FG fibers in the rectus femoris muscle (P <.05). Steers administered SbV had larger SO, FOG, and FG fibers in the psoas major muscle and SO fibers in the supraspinatus and semitendinosus muscles (P <.05). The combined administration of SYN and SbV had minimal, if any, effect on the percentage distribution of fiber types (P >.05) but increased (P < .05) the fiber areas of all muscles (18.5 to 54.8%) except the rectus femoris (P >.05). Proximate composition of the muscles was generally not affected (P >.05) by any of the treatments. The only observations were decreases in fat content for psoas major and rectus femoris muscles as a result of the combined administration of SYN and SbV. These results indicate that both growth-promoting agents, SYN and SbV, have a potential to increase muscle fiber size, but muscles respond differently to the administrations of SYN and SbV. However, when SYN and SbV are administered in combination, the combined effects result in an additive increase in muscle fiber hypertrophic response.