Recent studies have shown reduced 18:1 fatty acids in brain total lipid of piglets fed formula with 17% 18:1 fatty acids rather than sow milk providing 37% 18:1. Because 18:1 is a major fatty acid in brain myelin lipid and is rapidly deposited during myelination, it is important to know if this reflects deposition of myelin lipid containing reduced 18:1 and/or delayed myelination, or is related to changes in other brain membranes. Therefore, these studies determined the myelin total lipid, phosphatidylethanolamine and plasma and liver phospholipid fatty acids in piglets fed from birth to 15 d with formula containing (g/100 g fatty acids): 18:1, 17 and 18:3(n-3), 0.8; 18:1, 38 and 18:3(n-3), 0.6 fatty acids; or 18:1, 40 and 18:3(n-3), 4; or sow milk. Different levels of formula 18:3(n-3) fatty acids were studied because of the known effect of essential fatty acid deficiency on myelination. The brain 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNPase) activity and cerebroside concentration were also determined and used as indicators of myelination. Piglets fed the low 18:1 formula had lower 18:1 fatty acids in their plasma and liver phospholipid than sow milk-fed piglets. Formula providing a similar level of 18:1 to sow milk resulted in higher 18:1 fatty acids in the piglet plasma and liver phospholipid than in the sow milk-fed piglets. The brain myelin monoenoic fatty acids, CNPase activity and cerebroside concentration were similar in sow milk- and formula-fed piglets, irrespective of the formula 18:1 or 18:3(n-3) content.(ABSTRACT TRUNCATED AT 250 WORDS)