Supplementation with a high dose (600 g/d) of rumen inert conjugated linoleic acids (RI-CLA) inhibits milk fat synthesis in total mixed ration (TMR)-fed dairy cows immediately post partum. However, effects of RI-CLA on milk fat and bioenergetic parameters during the transition period in grazing cows have not been investigated. Multiparous Holstein cows (n=39) grazing pasture were randomly assigned to one of three treatments: (1) pasture (PAS), (2) PAS+540 g/d Hyprofat (palm oil; HYPRO) and (3) PAS+600 g/d RI-CLA. HYPRO and RI-CLA supplements were isoenergetic, fed twice daily at 7.00 and 16.00 and provided 0 and 125 g CLA/d, respectively. Treatments began 27±10 d prepartum and continued until 36±1 days in milk (DIM). There was little or no overall effect of RI-CLA on content or yield of milk protein and lactose. RI-CLA supplementation decreased overall milk fat content and yield with RI-CLA-induced milk fat depression (MFD) becoming significant by day 3 when compared with PAS and by day 6 when compared with HYPRO. MFD continued to increase in severity during the first 24 d post partum after which MFD reached a plateau (~40%; RI-CLA v. HYPRO). Pasture-fed cows produced less milk (19·4 kg/d) than the lipid-supplemented groups and although there were no overall differences in milk yield between RI-CLA and HYPRO (22·3 kg/d) a curvilinear relationship (R2=0·57) existed between the RI-CLA-induced milk yield response and extent of MFD. RI-CLA tended to increase milk yield (1·8 kg/d) compared with HYPRO until MFD exceeded 35% (~day 21), after which point the positive milk yield response was eliminated. Milk fat trans-10, cis-12 CLA content averaged 0·25 g/100 g in the RI-CLA treatment, was temporally independent, and was undetectable in PAS and HYPRO treatments. Based on the milk fat 14[ratio ]1/14[ratio ]0 ratio, RI-CLA decreased the overall Δ9-desaturase system compared with PAS and HYPRO. Compared with HYPRO, RI-CLA had no effect on plasma glucose, insulin, leptin, or NEFA concentrations. Results indicate that a high RI-CLA dose decreases milk fat synthesis and tends to increase milk yield immediately post partum in pasture-fed cows; however, excessive MFD (>35%) appears to be associated with a diminished milk yield response.