Rye is an important source of plant lignans in Finland. In the present crossover trial we wanted to study the effect of rye bread as part of the usual diet on serum and urine enterolactone (ENL) concentrations in healthy volunteers. Eighteen men aged 43 (SEM 2·0) YEARS AND TWENTY-ONE WOMEN AGED 43 (sem 1·6) years consumed wholemeal rye bread and white wheat bread in random order for 4 weeks. The bread periods were separated by a 4 week wash-out period. The breads provided at least 20 % of the daily energy intake. The mean intakes of rye bread were 219 (sem 14·6) and 162 (sem 5·3) g/d and those of wheat bread were 200 (sem 9·6) and 153 (sem 5·8) g/d for men and women respectively. Blood samples were collected from all subjects and three 24 h urine samples were collected from ten men and twelve women at the end of both bread periods for the determination of serum concentration and urinary excretion of ENL. The mean serum ENL concentrations in both men and women at the beginning of baseline period and at the end of the rye-bread period remained constant and were significantly higher than those at the end of the wheat-bread period. Correspondingly, daily urinary ENL excretion increased significantly during the rye-bread period compared with the wheat-bread period and was 5- and 10-fold higher in men and women respectively in comparison with the amount of plant lignan precursors measured in the rye bread. These data indicate the presence of other precursors for ENL in rye which are not detected by the current method of measuring plant lignans in food. The possible role of fibre in enhancement of the formation of mammalian lignans from their plant precursors in the gut also remains to be determined.