In vitro studies indicate that relative amounts of apolipoproteins CII and CIII in plasma may alter lipoprotein lipase mediated triglyceride removal from very low density lipoprotein. Therefore, we have determined relative amounts of these peptides in pregnant women in late gestation (mean 36 weeks) and 6 and 20 weeks postpartum. Very low density (d less than 1.006) and intermediate density (d 1.006-1.019) lipoproteins have been examined because they are affected similarly in pregnancy. Triglyceride and cholesterol in these two fractions increased 3 1/2 to 4 1/2-fold in pregnancy in keeping with previous reports. On a relative basis, apo CII was decreased and apo CIII1 and CIII2 increased in both VLDL and IDL at 36 weeks gestation. At 6 and 20 weeks postpartum relative amounts of C peptides returned to identical levels and were unaffected by lactation. It remains to be seen if in vivo triglyceride removal in pregnancy is altered by the reduction in apo CII relative to apo CIII.