the incidences of all primary neoplasms in the testes and accessory reproductive organs of 51,230 male fischer 344 ( f344 ) rats and 46,752 male b6c3f1 mice were obtained from the pathology data base of more than 300 long - term toxicity / carcinogenicity studies performed for the national toxicology program . 
 the overall incidence of reproductive system neoplasms in male f344 rats was 81.5% . 
 the most common neoplasms were interstitial cell adenoma of the testis ( 76.6% ) , adenoma / carcinoma of the preputial glands ( 2.9% ) , mesothelioma of the tunica vaginalis ( 1.5% ) , and adenoma of the prostate gland ( 0.3% ) . the combined incidence ( 0.1% ) of 59 other rare neoplasms consisted of 18 different types that occurred with a frequency of 4 or less . 
 in contrast to the rats , male b6c3f1 mice had a low overall incidence ( 0.6% ) of neoplasms in reproductive organs . 
 the most common neoplasm was the interstitial cell adenoma ( 0.4% ) . 
 thirty additional types of neoplasms were identified . 
 each of these uncommon neoplasms occurred at a frequency of 12 or less with an overall combined incidence of 0.2% . 
 morphological features are described for neoplasms in the rat and mouse , and criteria are presented for differentiation of testicular interstitial cell adenoma , prostatic adenoma , and mesothelioma from hyperplastic changes.imagesfigure 1.figure 2.figure 3.figure 4.figure 5.figure 6.figure 7.figure 8.figure 9.figure 10.figure 11.figure 12.figure 13.figure 14.figure 15.figure 16.figure 17.figure 18.figure 19.figure 20.figure 21 .