Production rates (entry rate into blood plasma) and other metabolic parameters for the IgG1 and IgG2 subclasses of immunoglobulins in mammary secretions (ratio of about 7 to 1) were determined in cows around the time of parturition by both single-injection and continuous-infusion isotope-dilution techniques. Four cows were given a single dose of 150 to 200 muCi of iodine-125 labeled IgG1 and 100 to 250 muCi of iodine-131 labeled IgG2 at 2 to 1 wk before parturition. Four cows, including two of the above, were infused continuously with constant amounts of the labeled immunoglobulins starting at 11 to 4 days before parturition. All cows were maintained until 4 to 6 days after parturition for monitoring the specific activities of iodine-125 labeled IgG1 and iodine-131 labeled IgG2 in the plasma and mammary secretions. Maximum entry rates of IgG1 and IgG2 were between 3 and 1 day prepartum with means of 125 and 60 g/500 kg body weight per day. The exchangeable pool means for IgG1 and IgG2 were 619 and 643 g/500 kg body weight, and both immunoglobulins were divided almost equally between the intra- and extravascular pools. A greatly increased production and a shortened half-life or greater turnover for plasma IgG1 occurs around the time of parturition which can account for the large accumulation of IgG1 in colostrum.