The pathogenesis of 210Po-induced tumors in the peripheral lung of Syrian golden hamsters was studied in a serial sacrifice experiment utilizing plastic (glycol methacrylate) and routine paraffin embedding procedures for lung sections. A rapid progression from hyperplasia of bronchiolar-type cells that appear in the alveolar region to malignant tumors was documented. Tumors began appearing as early as 15 wk after the 1st intratracheal instillation of 210Po.