A study was conducted to determine whether protected tuna oil can be used to enrich sheep's milk with eicosapentaenoic acid (EPA, 20:5) and docosahexaenoic acid (DHA, 22:6) without affecting milk yield or composition. Sixteen sheep in late lactation were selected from a dairy herd and assigned into two groups: Control (C) and protected tuna oil (PTO)-supplemented group. The two groups were run in adjacent paddocks where both had ad libitum access to oaten hay. Both were group-fed lupins (500g per animal per day) in a trough after morning (06:00h) and afternoon (15:00 h) milking. At morning milking the PTO group were offered a mixture of chaff (160 g) and molasses-coated (80:20, w/w), protected tuna oil supplement (150 g). At each milking, the control group were offered 50-100 g of a mixture of oaten chaff (0.70) and lupins (0.30) for contentment during milking. Supplement feeding lasted 10 days and milk samples were collected from Day 0 (pre-feeding) to 6 days post-feeding period. Tuna oil supplementation did not affect daily milk yield (722 +/- 81 ml versus 716 +/- 80 ml), fat (74 +/- 8.3 g/kg versus 77 +/- 0.8.6 g/kg) or protein percent (54 +/- 6.0 g/kg versus 56 +/- 6.3 g/kg; C versus PTO, respectively). No apparent residual effect on milk yield or composition was detected 6 days post-feeding period. The levels of EPA and DHA in milk fat from the PTO group increased linearly from Day 2 (0.7 +/- 0.7 g/kg and 1.7 +/- 0.6 g/kg) up to Day 6 (3.2 +/- 0.8 g/kg and 170 +/- 1.8 g/kg) and tended to plateau thereafter (Day 10 values: 4.7 +/- 0.5 g/kg and 19 +/- 0.9 g/kg, respectively). Six days after the supplement-feeding period, the milk from the PTO group was still significantly enriched with n-3 PUFA (EPA: 2.3 +/- 0.9 g/kg; DHA: 6.5 +/- 0.9 g/kg). No EPA or DHA was detected in milk samples from the non-supplemented group. The apparent transfer of dietary EPA and DHA to milk was 0.21 +/- 0.02 for EPA and 0.18 +/- 0.03 for DHA (computed using milk samples from Day 10). A cup (250 ml) of such n-3 PUFA-enriched milk will provide two-thirds (437 mg, 0.67) of the recommended daily intake of EPA plus DHA (650 mg) for adults. This study provides an approach that enables enrichment of milk with health-enhancing fatty acids without deleterious consequences on milk yield or composition. Crown Copyright (C) 2003 Published by Elsevier Science B.V. All rights reserved.