The potential benefits on human health have prompted an interest in developing nutritional strategies for specifically increasing rumenic acid (RA) in ruminant milk. The aims of the present study were to (i) compare two dietary treatments with lipid supplements on milk yield and composition, (ii) measure the in vivo Delta-desaturation of vaccenic acid (VA) to RA using (13)C-labelled VA and (iii) determine the effect of the dietary treatments on this variable. Treatments were 90g sunflower-seed oil (SO) per d or 60g sunflower-seed oil and 30g fish oil per d plus additional starch (SFO), in a grassland hay-based diet given to eight Alpine goats in a 2 x 2 cross-over design with 21 d experimental periods. Milk yield and composition were similar between treatments. Goats fed SFO had higher milk 6 : 0-16 :0 concentration, lower milk Sigma C18 concentrations and showed no effect on milk VA and RA, compared with SO. At the end of the experiment, intravenous injection of 1.5 g [(13)C]VA followed by measurements of milk lipid (13)C enrichment showed that in vivo 31.7 and 316% of VA was Delta 9-desaturated into milk RA in the caprine with the SO and SFO treatments, respectively. The expression of genes encoding for Delta 9-desaturase (or stearoyl-CoA desaturase; SCD1, SCD5) in mammary tissues and four milk Delta-desaturation ratios were similar between treatments. In conclusion, the present study provides the first estimates of in vivo endogenous synthesis of RA (63-73 % of milk RA) from VA in goats, and shows no difference between the two lipid supplements compared.