The objectives of this study were to evaluate ruminal degradation and intestinal digestion of two ruminally protected methionine (RPM) products and to assess the potential use of changes in plasma methionine concentrations as an indication of methionine availability to the animal. Ruminal degradation of the protected methionine was assessed using the in situ technique. The intestinal availability of methionine after ruminal incubation was determined in vitro using an enzymatic procedure. Four Holstein cows receiving a typical mid-lactation ration (16.5% CP, 1.6 Mcal NE(L)/kg) were supplemented with 0, 30, and 60 g per day of a slowly degraded ruminally protected methionine (SDM), or 60 g per day of a moderately slowly degradable ruminally protected methionine (MSDM) in a 4x4 Latin square design. Blood samples were collected from the jugular vein at 0, 6, 12, 18, 24, and 36 h after feeding the RPM sources. Ruminal degradation rates of SDM and MSDM were 0.03 h-1 and 0.07 h-1, respectively. The calculated amount of methionine available for absorption, based on the in situ and in vitro results, was 17.9, 11.9 and 23.8 g per day when dosing 60 g of MSDM, 30 and 60 g of SDM, respectively. The highest (p<0.05) methionine plasma concentration (133.9 μM) was measured with 60 g of SDM, followed by 30 g of SDM, and 60 g of MSDM. Plasma methionine concentrations were affected by an interaction (p<0.05) between time after dosing methionine and rate of ruminal degradation of the methionine dosed. Methionine plasma concentration peaked 12 h after dosing SDM, whereas methionine plasma concentration appeared to peak between 6 and 12 h after feeding MSDM. There was a good relationship (r2=0.86) between the grams of methionine escaping from the RPM products and the greatest area under the curve describing plasma methionine concentration. Data from this study show that the lower the ruminal degradation rate, the later the maximum plasma concentration of methionine will occur, and that the plasma methionine concentrations or their area under the curve can be successfully used to predict bioavailability of ruminally protected methionine. (C) 2000 Elsevier Science B.V.