Previously, this laboratory has found very high concentrations of IgD in normal rat milk. Using ELISA methods, the relationship between milk, serum and saliva IgD in lactating and suckling rats was examined. Milk IgD appears to be synthesized in the mammary tissue rather than taken up from the blood because (i) serum IgD remains low and is not significantly different from that of non-lactating females, and (ii) serum IgD during lactation is poorly correlated with milk IgD. Serum IgD in suckling rats declines in the first 7 days following birth and remains relatively low during the remainder of lactation (2-4 micrograms/ml). The surprisingly high serum IgD observed at birth (9.3 +/- 3.2 micrograms/ml) is present before suckling begins and is not affected by the onset of suckling. Transient elevations of serum IgD begin to occur following weaning. Rats weaned 10 days earlier than normal (Day 20 vs Day 30) had significantly higher serum IgD on Days 36, 47 and 60. Among 43 adult rats, serum IgD was 5.4 +/- 3.6 micrograms/ml and saliva IgD 2.0 +/- 1.7 micrograms/ml. Serum IgD correlates poorly with saliva IgD. The thymus is not required for IgD synthesis since no significant difference in serum IgD was found between nude rats and their euthymic littermates.