Background : Familial adenomatous polyposis (FAP) is a rare disease affecting ~1 in 10,000 individuals and accounts for <1% of colorectal cancer cases. Untreated FAP carries a nearly 100% lifetime risk of colorectal cancer, typically manifesting by age 40 and is treated by colon resection between 15-25 of age. The Pirc ( p olyposis i n r at c olon) model of FAP harbors an Apc gene mutation analogous to that in humans. This model develops multiple polyps in the colon and duodenum with age and is widely used to test preventive interventions and in mechanistic studies. Longitudinal monitoring of polyp number, size, and grade is essential for preclinical testing of efficacy of preventive interventions and can be achieved through colonoscopy. In this study, we compared rigid endoscope-enabled colonoscopy with postmortem analysis of polyps in Pirc rats. Methods : Colons of 16-week-old male and female Pirc rats (n=8) were examined using a rigid endoscope (Small Coloview scope, 10 cm, Karl Storz, Tuttingen, Germany). Before colonoscopy, rats were anesthetized with isoflurane, colons were flushed with warm water to remove fecal matter. In the video review, polyps were graded based on the degree of lumen obstruction: G1: barely detectable; G2: ≥ barely detectable to <1/8 th of lumen; G3: ≥1/8 to <1/4 of lumen; G4: ≥1/4 to <1/2 of lumen; and G5: ≥1/2 of lumen to complete blockade. Animals were then euthanized, and colons were excised to determine polyp number and diameter measured with digital caliper (General® Ultratech, CA, USA). Total and detectable polyp counts were compared between the two methods using Wilcoxon matched-pairs signed rank test. Mean ± standard deviation (SD) of polyp diameter for each polyp grade was reported. Statistical analyses were performed using GraphPad Prism v.10. Results : The median total polyps count was 2 (range: 0-5) using the endoscope and 3 (range: 1-12) postmortem, with no statistically significant difference (p= 0.09). Because the rigid endoscope visualizes only the distal 10 cm of rat colon, detectable polyps were compared. The median detectable polyps count was 2.5 (range: 0-7) postmortem and was not significantly different from colonoscopy (p= 0.5). The mean ± SD of the polyp diameter was as follows: G2: 2.9±0.7 mm, G3: 3.4±0.5 mm, G4: 3.6±0.6 mm, and G5: 3.6±0.7. Limitations of colonoscopy included incomplete visualization of the entire colon, difficulty distinguishing adjacent polyps, and challenges in detecting rectal polyps. Conclusion : Polyp detection using a rigid endoscope is feasible and comparable to postmortem evaluation, supporting its use for longitudinal monitoring in preclinical prevention studies. Flexible and extendable endoscopes may enhance polyp detection and monitoring. Acknowledgements: This research was funded by CPRIT RP240401. Dr. Rashim Singh is supported by the NCATS/NIH under award number K12TR004522.