We examined the effects of cellophane wrapping of the pancreas on the age-related uptake of tritiated thymidine (3H-TdR) by the differentiated cell types of the pancreas of the Syrian golden hamster. Fifty-two hamsters were studied. At 7 weeks of age, hamsters underwent cellophane wrapping (n = 32) or were allocated to a control group (n = 20). Animals 8-22 weeks of age (four at each interval) received 3H-TdR (2 .mu.Ci/g) intraperitoneally and were killed 1 h later. Pancreatic tissue from each animal was processed for autoradiography. The percent of acinar cells labeled with 3H-TdR at 8 weeks, in control and wrapped animals, was 1.17 .+-. 0.26 and 1.51 .+-. 0.38 respectively (p = N.S.). In control animals, this steadily diminished to 0.02 .+-. 0.00 at 22 weeks. In wrapped animals, there was less of a tendency for acinar cell labeling to decrease with age, and the percent of labeled acinar cells in wrapped animals at 22 weeks was 0.05 .+-. 0.00. The percent of ductular cells labeled with 3H-TdR at 8 weeks in control and wrapped animals was 0.24 .+-. 0.24 and 0.98 .+-. 0.24, respectively (p = N.S.), and at 22 weeks was 0.13 .+-. 0.90 and 0.60 .+-. 0.03, respectively (p < 0.01). The percent of islet cells labeled with 3H-TdR at 8 weeks in control and wrapped animals was 0.16 .+-. 0.01 and 0.42 .+-. 0.01, respectively (p < 0.05), and at 22 weeks was 0.18 .+-. 0.01 and 0.63 .+-. 0.05 (p < 0.05), respectively. In the major ducts in the head of the gland, the number of epithelial cells per millimeter duct length that were labeled with 3H-TdR in control animals was stable throughout the study: 3.25 .+-. 1.5 at 8 weeks and 3.35 .+-. 1.3 at 22 weeks. In the wrapped group, the number of labeled duct epithelial cells was greatest at 8 weeks (22.5 .+-. 7.5) and then declined to 13.7 .+-. 4.5 at 12 weeks, at which time it stabilized. It is concluded that cellophane wrapping of the pancreas is a mitogenic stimulus that affects all cell types in the pancreas of the Syrian golden hamster. The greatest trophic affect was on cellular elements of the ductal system and on islet cells.