During lactation, the concentrations of Ca and P in milk are highly conserved, frequently at the expense of bone mineral. Replacement of bone mobilized during lactation and dietary changes in the post- weaning period may further alter Ca and P metabolism, but it is unknown if these changes influence the ability to achieve Ca and P homeostasis. The objective of this study was to examine changes in plasma Ca and P in lactating mares fed decreasing amounts of Ca and P just before weaning. Four mares in late lactation (12 ± 5.7 yr; 527 ± 26 kg) were transitioned to a diet containing Ca and P in amounts appropriate for nonlactating mares for 14 d, weaned and then maintained on the same diet for 7 d. Four nonlactating mares (11 ± 4 yr; 552 ± 33 kg) received the same diet and served as the controls. We hypothesized that low mineral intake during late lactation would stimulate bone resorption (increasing blood Ca and P) and that in the post-weaning period there would be bone deposition (decreasing blood Ca and P). Blood samples for P and Ca analysis were obtained 14 d before weaning (dietary Ca and P equal to or exceeding requirements), at 2 d before, and 7 d after weaning (dietary Ca and P adequate for nonlactating mares). Saliva samples were taken 7 d post-weaning for P analysis. Data were analyzed using ANOVA with repeated measures with group (lactating or nonlactating) and sample date as main effects. Plasma P increased from baseline to pre-weaning (P < 0.05) across groups and did not change further post-weaning. The increase in plasma P at 2 d pre-weaning might reflect an increase in bone resorption as a result of decreasing amounts of dietary P, but a decrease in plasma P 7 d post-weaning was not observed. Salivary P was not different between groups post-weaning. Plasma Ca decreased from baseline to pre-weaning (P = 0.053) across groups, but did not change further post-weaning. The Ca:P ratio in the blood decreased from 3.7:1 at baseline to 2.8:1 at pre-weaning (P < 0.05). From these data, it appears that changes in Ca and P intakes influence blood Ca and P more than the physiological change of lactation cessation.