Methods Animals. Male Sprague-Dawley rats obtained from two dif-ferent breeders (Lippische Versuchstierzucht, Extertal and Zentralinstitut für Versuchstiere, Hannover, Federal Republic of Germany) weighing 190 to 220 g were used in these experi-ments. The animals had free access to standard laboratory chow and water for 2 weeks at our institution before the experiments were started. They were kept on an automatically controlled light and dark schedule. Forty-four rats from the Extertal breeder were used for the first set of experiments. These studies were repeated with 44 rats supplied by the Hannover breeder. The experimental protocol is shown in Figure 1.

Labeling Flash labeling. In each series 24 rats received a single intravenous injection of 3H-TdR in a lateral tail vein while the animals were kept in a restraining cage. The dose was 1 Ci/g body weight (sp act 5 Ci/mM, Amersham Buchler, Braun-schweig, Federal Republic of Germany). The rats were injected in the morning (8 to 10 AM) to avoid the influence of circadian variations of the cellular growth cycle [11]. Groups of four rats were anesthetized with ether after 2 hr, and 1, 3, 5, 7, and 14 days, respectively. The abdomen was opened by a midline incision. The aorta was clamped above and below the renal arteries and the superior mesenteric artery was also occluded. The kidneys were flushed with 10 ml of Ringer solution via the aorta while the inferior vena cava was opened by incision. Then 20 ml of 2% glutaraldehyde in 0.1 M cacodylate buffer (pH 7.3)