Locoweed (Astragalus lentiginosus) fed to mature rams induced transient degenerative changes in the seminiferous, epididymal and vas deferens epithelium. There were changes in behavior and loss of condition in the treatment rams. Observed libido was reduced. Estimated percent abnormal sperm was significantly (P less than 0.05) higher in the locoweed-fed rams than in the controls. Increased retained proximal cytoplasmic droplets, separation of the heads and tails of the spermatozoa, and reduced sperm motility were observed. Serum SGOT (serum glutamic oxaloacetic transaminase) levels were significantly (P less than 0.01) elevated, and WBC (white blood cell) counts were decreased (P less than 0.01). Grossly no changes were observed. Histologically there were advanced lesions of foamy cytoplasmic vacuolation in all tissues taken at necropsy, but particularly in the epithelium of the seminiferous tubules, epididymis and vas deferens. Reduced spermatozoa production was evident. Testicular changes were transient and rams appeared clinically normal 70 days after locoweed feeding was terminated; however, there was slight residual atrophy of the germinal epithelium at necropsy.