Hypervitaminosis A and D is a potential cause of "hyena disease" in cattle, which results from premature growth-plate closure in long bones of calves. This study showed that vitamin A induced growth-plate closure if calves were given an intramuscular injection of vitamins A and D (2,000,000 IU and 300,000 IU, respectively) on the first day after birth and, in addition, vitamin A (30,000 IU/kg body weight) in a water dispersible form was added to the milk substitute daily. Gross lesions were observed in the proximal tibial growth plates of each of seven calves after 3 weeks of vitamin-A treatment. Microscopical examination showed commencing premature growth-plate closure in the proximal tibia at 2 weeks. After one week, the growth plate showed focal thinning, and there was premature endochondral ossification of columnar cartilage. Longitudinal bone growth was dramatically reduced before growth plate closure at one week (25 microns/day in a treated animal versus 136 microns/day in a control). Liver concentrations of retinol and retinyl palmitate became strikingly elevated at on week, and thereafter increased slowly until the third week. Elevation of plasma retinol and retinyl palmitate was rapid, reaching a maximum on day 10. Plasma all-trans-retinoic acid was undetectable in many samples from treated animals, but plasma concentrations of derivatives of retinoic acid (9-cis-retinoic acid, 13-cis-retinoic acid, 13-cis-4-oxoretinoic acid, and 9, 13 dicis-retinoic acid) were elevated. The vitamin-A intake required to induce growth-plate closure in calves was found to be exceedingly high. Vitamin-A toxicity must be considered as a potential cause of hyena disease, but it would seem likely that other factors also play a role.