The feeding, ruminal effects and potential toxicity of cull onions bulbs (Allium cepa) was compared to whole sorghum grain for growing wethers. In Expt. 1, 56 Polypay X Rambouillet wethers (avg. initial BW of 32.7 ± 0.88 kg) were assigned randomly to one of four dietary treatments (dry matter basis): (1) 50% whole sorghum grain (CON); (2) 33% sorghum:17% onions (17%); (3) 17% sorghum:33% onions (33%); and (4) 50% onions (50%). The remaining 50% was pelleted alfalfa (19% CP). All wethers were group fed (seven wethers per pen; two replicates per treatment) for 6 weeks. Weight gain during the trial did not differ (P> 0.05) among treatments. Packed cell volume tended to be less (P = 0.057) for the 33% and 50% onion groups during weeks 1 and 2, and was less (P< 0.0001) for the same groups during week 3 (34, 33, 29 and 29% for CON, 17, 33 and 50%, respectively). Serum lactate dehydrogenase was increased (P< 0.05) at weeks 3 and 6 in sheep on the 33% and 50% onion treatments. During Expt. 2, 15 ruminally cannulated wethers were individually fed diets similar to those Expt. 1, except onions were fed to replace sorghum grain to provide 0, 25 and 50% of the dietary DM as onions. Ruminal fluid (P> 0.05) and particulate passage (P> 0.05) were not altered by feeding onions. Water intake by drinking decreased (P< 0.0001) as percentage of onions in the diet increased, whereas total water intake (drinking and fed) increased (P< 0.0001). Effects of onions on volatile fatty acid concentrations and ruminal pH were minimal (P< 0.05), whereas ruminal ammonia-N concentration was greater (P< 0.05) in wethers fed onions. We conclude that under conditions similar to those in our study, onions can be fed safely to sheep with weight gains comparable to those from whole sorghum grain.