Plasma calcium and inorganic phosphorus concentrations were significantly lower in 14 female rabbits during the last 20 days of pregnancy and the first 20 days of lactation than in 11 non-pregnant, non-lactating controls. Plasma magnesium level was similar in both groups. Plasma calcium and inorganic phosphorus levels were very low (5.8 +/- 0.4 and 2.9 +/- 0.3 mg/dl, respectively) in 6 lactating rabbits afflicted with tetany between 7 and 18 days postpartum. When they were intraperitoneally injected with calcium, the calcium and phosphate levels returned to normal within 2 h.