Rat immunoglobulin A, M, G2a and G2b and anti-rat α, μ, and γ were purified by ion exchange and immunoadsorbent chromatography. These purified immunoglobulins and antisera were employed in the quantitation of the principal immunoglobulins in serum and saliva of young rats from birth until 45 days of age and in milk, saliva and serum of adult rats. The level of serum IgG2a in young rats rapidly increased prior to weaning (days 15-19) and approached the adult level (6·32 mg/ml). This increase in pup serum IgG2a was apparently due to passive transfer since, (a) an increase in IgG2a was observed in rat colostrum and milk between day 0 (0·67 mg/ml) and day 19 (1·53 mg/ml) and (b) upon weaning the level dropped and gradually increased to a level of 4·05 mg/ml by day 45. Serum IgM was first detected in 10 days old rats and approached the adult level (·82 mg/ml) this reached the adult level (1·19 mg/ml) by day 30. Adult levels of salivary IgG2a (0·06 mg/ml protein) was observed in 19 day old rats. However, salivary IGA was not detectable until day 25 and this Ig approached the adult level (0.·04 mg/mg protein) by day 35. No IgM could be detected in rat colostrum, milk or saliva. © 1975.