Our previous study has revealed that providing dry feeds increased the plasma concentration of antidiuretic hormone (ADH) in suckling calves, leading to altered water balance. To examine whether ketone bodies formed from ruminal fermentation-derived butyrate induced ADH secretion in suckling calves, the effects of intraruminal administration of butyrate on plasma concentration of ADH and ketone bodies, plasma and urine osmolality, and urine volume were examined. Six male Holstein calves aged 4 wk were used. Three levels of butyrate (0 g, 22 g and 44 g) were intraruminally administrated in a 3 x 3 Latin square design, and blood plasma and urine were analyzed. Plasma concentration of ketone bodies was increased by intraruminal administration of butyrate within 15 min in a dose-dependent manner, and the elevation of plasma levels continued until 4 h. Plasma concentration of ADH was also increased by the butyrate treatment, and it was higher in the 44 g butyrate group than in the 22 g butyrate group from 15 min to 2 h. The duration of the elevated plasma concentration of ADH was shorter than that of plasma concentration of ketone bodies. The relationship between plasma concentrations of ADH and those of ketone bodies was statistically significant, although the relationship was weaker. In accordance with the elevation of plasma ADH levels, the butyrate treatment resulted in the decreases in urine volume and increases in urine osmolality. Plasma osmolality was not different among the groups. The present results suggest that ruminal butyrate-derived ketone bodies are at least partly responsible for ADH secretion in suckling calves fed with dry feeds, and that the secreted ADH decreases urine volume through the increase in urine osmolality. (c) 2006 Elsevier B.V. All rights reserved.