Serum inorganic phosphorus (Pi) concentration of 20 cows in each of ten factory supply dairy herds was assessed at monthly or two-monthly intervals during the 1982–83 lactation. Pasture on offer was ranked low, medium or high and the phosphorus (P) content assessed monthly on all farms from the two paddocks to be grazed next in the rotation. The mean serum Pi concentration was high (1.98 mmol/1) prior to calving but fell to low levels at peak lactation (1.28 mmol/1) and again during the drought (1.28–1.38 mmol/1) in January, February and March. Individual cows had Pi levels as low as 0.32 mmol/l. Herds on Northern Yellow-brown Earths had higher Pi levels than herds predominantly on Brown Granular Loams (P<0.01). There were differences between cows within herds (P<0.01) and Pi levels declined with cow age (P<0.01). Pasture P content was above minimum requirements for lactating cows (0.33% DM; withad lib. feeding) from July through October but below requirements in most pastures from December through April, when pasture availability also limited production. The P content in pasture was unrelated to either its grass or legume content, but was higher in pastures given a medium or high DM ranking (P<0.05). The possibility of increasing dairy production with P supplementation in spring is discussed.