Serum inorganic phosphorus (Pi) and calcium (Ca) concentrations were assessed in 20 cows on each of four Manawatu factory supply dairy farms. Blood was taken from each cow before calving and at six-week intervals during lactation. Bleeding coincided with herd testing. Herds of Friesian or Friesian X cows and Jersey or Jersey X cows were compared on adjacent farms on a Central Yellow-brown Sand and on adjacent farms on a Peat Loam overlying a Central Yellow-brown Earth soil. Pasture mass and composition were estimated to grazing height in the next two paddocks to be grazed in the rotation. Mean serum Pi concentration was higher in cows on sandy soils (1.55 mmol Pi/ℓ) than in cows on the peat loam (1.34 mmol Pi/ℓ (P<0.001). Concentrations were highest before calving (1.69 mmol Pi/ℓ) but fell to low levels at peak lactation (1.17mmol Pi/ℓ) when 70% of cows were below the minima of the ‘normal range’, and during the drought (1.29 mmol Pi/ℓ). Pasture phosphorus (P) concentrations were adequate to support cow nutritional P requirements for lactation (>0.33% DM,ad lib. feeding) until the summer drought when low herbage mass would have restricted milk production. Serum Ca was adequate for lactating cows and changed little between months or between cows (mean 2.12 mmol Ca/ℓ). No metabolic disorders relating to mineral deficiencies were observed. It appears that serum Pi in a high proportion of cows falls below the normal range during peak lactation without cows displaying clinical deficiency symptoms or a depression in butterfat production.