Changes in pancreatic growth and in mRNA concentrations in rat pancreas were monitored by dot-blot hybridization with cloned cDNAs of rat amylase, chymotrypsinogen B, proinsulin I, and actin during the pre- and postnatal period in the rat. Wistar rats were killed at the 18th day of gestation and at the 1st, 10th, 20th, 35th, and 87th day of postnatal life. It was concluded from the ratio of pancreatic weight/body weight that pancreatic growth preceded body growth. Pancreatic protein and total RNA concentration increased 2.9 times during the period studies. All studied mRNAs increased in concentration during the postnatal development period. Messenger RNA for chymotrypsinogen B and proinsulin I exhibited a significant increase after birth, decreased by the 10th day of life, and increased thereafter. For amylase mRNA, no significant changes were observed around birth, a progressive increase occurring thereafter up to the 87th day of life. The mRNA for actin showed a progressive increase between the 18th day of gestation and the 20th postnatal day, after which it remained stable. We concluded that each mRNA showed a singular profile of increase during postnatal development.