Thirty-two crossbred wether lambs were assigned to a feed intake level of either ad libitum (ADLIB) for maximum rate of growth or restricted to maintain body weight (MAINT) throughout a 21-d period. At 7-d intervals (d 0.7, 14, and 21), four lambs per treatment were slaughtered to obtain measurements of visceral organ protein synthetic capacity and tissue composition. Protein synthetic capacity was assessed by in vitro [14C]valine incorporation and tissue RNA, DNA, and protein contents. Concentrations of protein and RNA were not significantly affected in most tissues measured. However, for liver, duodenal, and jejunal tissue, DNA concentrations in ADLIB lambs were lower (P less than .05) than in MAINT lambs. Ratios of protein:DNA in most organs were higher (P less than .05) in ADLIB than in MAINT lambs. During the 21-d period, liver and small intestinal protein and RNA mass were higher (P less than .10) in ADLIB than in MAINT lambs, and DNA mass was unaffected. Also during the 21-d period, the average total mass of ruminal protein, RNA, and DNA in ADLIB lambs was higher (P less than .05) than in MAINT lambs. Estimates of valine incorporation and ratios of RNA:protein seemed to reflect protein synthetic capacity of the visceral tissues measured; however, the effect of level of feed intake on these measurements was equivocal. These data suggest that the level of feed intake affected visceral organ mass through changes in cellular hypertrophy.