Altogether 1645 blood plasma samples were collected from 2 reindeer (Rangifer tarandus tarandus L) herds in northern Norway (Magerøy and Sørøy), and from 2 herds in southern Norway (Filefjell and Lom) during the period from 1992 through 1995. Except for 2 subsets of samples from Lom (N = 51 and 56) all samples were collected on winter pasture between January and early March. The herds were of varying nutritional status, the Lom herd being regarded to be among the best in the country in this regard. Plasma levels of magnesium, calcium and inorganic phosphorus were measured. In addition, plasma progesterone was used as a pregnancy test, a discriminatory level of 7 nmol l-1 being chosen as indicating pregnancy. For the investigated minerals, the analysis of variance included effects for year of sampling, herd, pregnancy status, age and mineral status. Average mineral concentrations varied considerably between herds and year of sampling. The overall average (SD; min-max) concentrations of plasma Ca, Mg and P in samples collected on winter pastures were 2.42 (0.25; 0.9-3.6), 0.83 (0.17; 0.16-1.39) and 1.70 (0.47; 0.2-3.4) mmol l-1, respectively. The overall pregnancy rate was 79.8%. The frequency of subnormal plasma values within herds and years for magnesium (< or = 0.7 mmol l-1) and calcium (< or = 2.2 mmol l-1) varied between 0-61.9% and 1.4-44.9% respectively. Significant positive correlations between calcium and magnesium were found in all herds except in the Lom herd where all animals had plasma Mg values above 0.8 mmol l-1. Generally, the highest correlation coefficients were found in subsets of data with a high frequency of subnormal magnesium concentrations. Plasma magnesium showed the greatest contribution to plasma calcium variance when tested together with herd, year of sampling, pregnancy status, age and plasma inorganic phosphorus. A decrease in plasma magnesium from 1 to < 0.5 mmol l-1 was associated with a decrease in plasma calcium of approximately 15%. Mean plasma magnesium and calcium levels were significantly (p < 0.01) higher in pregnant animals than in barren females, a significant positive relationship being found between pregnancy rate and average concentrations of the same minerals when sets of observations from different herds and different years were compared. The positive correlation between plasma calcium and magnesium in herds with subnormal magnesium minima is consistent with evidence from other species reported in the literature of impaired calcium homeostasis in magnesium deficient animals.