Jugular blood samples were obtained from 55 summer-calving and 40 winter-calving Holstein cows fed diets which averaged 7.4 micrograms of Cu/g. Samples were collected within 3 days of 30, 11, and 4 days before calving and 4, 11, 18, 25, 39, 60, 88, 116, and 147 days after calving. Packed cell volume and plasma Cu concentrations were determined. Blood components were analyzed in relationship to season, milk production, lactation number, number of days to conception, number of postcalving uterine infusions, and time relative to calving. The environmental temperature-humidity index was used as a covariate in the analysis, so that seasonal effects represented feeding or production differences between seasons and were not due to the direct effects of climate. Plasma Cu concentrations were within the usual laboratory limits (1.05 +/- 0.20 microgram/ml). These values were unrelated to PCV in the summer-calving cows, but were slightly related in the winter-calving cows (r = 0.11). Plasma Cu values increased around the time of calving, with the maximum level (1.13 microgram/ml) occurring on postpartum day 11. Plasma Cu concentration was higher in cows that conceived within 80 days of calving than in cows which conceived at 120 days or more. Differences in plasma Cu among fertility groups were most evident during postpartum days 25 to 60. However, the relationship between plasma Cu concentrations and fertility was not consistent. Significant interactions existed for season X fertility groups and milk production groups X fertility groups. Plasma Cu concentration was significantly higher for the summer-calving cows and was inversely related to milk production level.(ABSTRACT TRUNCATED AT 250 WORDS)