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Notes: Two levels of lactic acid were added to a well-preserved silage to give silages with three concentrations of lactic acid: 53 (control), 96 (LA-50) and 134 (LA-100)g (kg DM)-1. These silages were given ad libitum with 5 kg d-1 of a mixture of barley and soya-bean meal (75:25) to six lactating dairy cows in a 6 × 6 Latin square design experiment to examine the effects on silage intake and milk production of the addition of lactic acid, both with and without a post-ruminal supplement of 230 gd-1 sodium caseinate. The six treatments were (1) control silage without casein, (2) control silage plus casein, (3) LA-50 silage without casein, (4) LA-50 silage plus casein, (5) LA-100 silage without casein and (6) LA-100 silage plus casein. Sodium caseinate was administered as a continuous infusion into the abomasum. Periods were 14d long.Both levels of addition of lactic acid reduced (P 〈 0·001) the intake of silage, values being 102, 94 and 86 kg DM d-1 for the control, LA-50 and LA-100 treatments respectively, but the casein supplement did not affect the intake of silage. Milk yield was reduced (P 〈 0·01) only for treatment LA-100 and, for this treatment, was restored (P 〈 0·01) by casein given post-ruminally. With all three silage treatments, casein infusion increased (P〈0·001) the yield of protein in milk by 30–35 gd-1. The concentration of protein in milk increased linearly (P 〈 0·001) with the addition of lactic acid to the silage.The addition of lactic acid produced a linear reduction (P 〈 0·001) in the molar proportion of acetate in rumen contents and compensatory linear increases (P 〈 0·001) in the molar proportions of propionate and butyrate. Changes during the day in the concentration of insulin in blood plasma followed a similar pattern to changes in the molar proportions of propionate. The mean daily concentration of insulin in plasma tended (0·10 〈 P 〉 0·05) to be higher for the LA-100 treatment, and in the immediate period after feeding this difference was significant (P 〈 0·05).The results confirm the effect of lactic acid in depressing the intake of silage, but they offer no support for the hypothesis that the depression of intake can be offset by an increase in the supply of protein post-ruminally.