Five sheep, with a rumen fistula inserted, were each injected (IV) with apomorphine (18 mg) and ruminal pH was measured every 5 minutes during a 1-hour period. During the base-line period (30 minutes) that preceded apomorphine injection, pH was constant in individual sheep and te group mean (+/- SD) was 7.1 +/- 0.35. After apomorphine was injected, group mean was 6.9, 6.0, 6.3, 6.3, 6.3, 6.4, 6.4, 6.5, 6.7, 6.9, 6.9, and 7.1 at 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, and 60 minutes, respectively. The pH reduction of ruminal content was recorded in 4 of the sheep. Maximal reduction occurred in the 10-minute period after apomorphine was injected. The pH returned to control value within 40 to 50 minutes after injection. Vomitus was not expelled through the mouth by any of the sheep. It was concluded that expulsion of acidic abomasal contents back into the preabomasal compartment (internal vomiting) was the cause of acidification of the rumen after apomorphine was injected.