As part of a nutritional study lasting from six weeks before calving to 22 weeks of lactation, blood samples collected from 47 dairy cows maintained under well-defined conditions were analysed for a variety of metabolites and hormones. At various times after the completion of the study, six of the animals developed clinical signs of bovine spongiform encephalopathy (BSE), although when they were sampled it was not known that they were incubating the disease. The data were used to make comparisons between the animals that developed BSE and those that did not develop the disease and which had been maintained under the same conditions. The greatest differences between the animals incubating BSE and the control animals were observed at times of nutritional stress, at the start of lactation and when the intake of concentrate feeds was reduced at week 13 of lactation. In the animals that subsequently developed BSE, feed intakes were lower in early lactation; plasma P-hydroxbutyrate concentrations were significantly higher (P<0.001) at weeks 3 and 5 of lactation; adjusted milk yields were lower until week 6 of lactation and milk fat concentrations were consistently lower. There was no effect on plasma glucose concentrations, although insulin concentrations were significantly lower in week 1 of lactation (2.27 v 2.50 μiu/ml) (P<0.05). The concentrations of plasma proteins and urea were unaffected by BSE incubation, apart from protein concentrations being significantly higher one week before calving, and the concentration of urea being significantly lower five weeks before calving. The plasma concentrations of somatotrophin, prolactin, oestradiol and progesterone were similar in the two groups of animals throughout the study. The differences observed indicate that the energy metabolism of dairy cows incubating BSE may be subtly altered before the onset of clinical signs of the disease.