1562 Background: The National Cancer Institute supports several national trial networks which responded rapidly to the COVID-19 pandemic to overcome operational barriers to clinical cancer research. The National Clinical Trials Network (NCTN) focuses on late phase treatment trials, while the Experimental Therapeutics Clinical Trials Network (ETCTN) conducts early phase treatment trials. We report findings on the experience and adaptations of these networks during COVID-19. Methods: Using 2019 and 2020 accrual data, we analyzed changes in accrual levels and demographics. We also evaluated changes in trial activation numbers and timelines. In July 2020, we surveyed 255 investigators from academic and community sites to assess changes in research practices and get feedback on modified processes implemented by NCI to address trial conduct during the pandemic. Results: Accrual across the NCTN and ETCTN fell significantly in mid-March 2020, dropping from a weekly average of 307 patients in February to 169 the week of March 23-29. Accrual began to recover in June and July but did not return to pre-pandemic levels until September. Accrual in November and December 2020 followed the patterns seen in 2019, with short-term drops around major holidays. Non-White participants were enrolled to NCTN and ETCTN trials at similar monthly rates throughout 2019 and 2020, with slightly higher overall enrollment in 2020 (23.7% vs. 22.7%). New trials continued to be developed and activated throughout 2020. Between 2017 and 2019, an average of 71 trials were activated per year (NCTN = 46, ETCTN = 25), compared to 84 activated in 2020 (NCTN = 58, ETCTN = 26). The average time to trial activation was similar or slightly longer in 2020 compared to 2019. The investigator survey yielded 111 responses (43.5% response rate). 43% of respondents’ sites paused enrollment to phase 1 trials during the pandemic, compared to 18% for phase 3 trials. Many sites temporarily stopped opening new trials and processing specimens. Sites were more likely to keep enrolling to trials offering clear potential benefit and pause complex trials that required more patient contact. Respondents attributed some of the decline in accrual to a reduction in overall patient volume, increased patient concerns, and reduced research staff on site. Respondents were asked to rate the usefulness of modified trial processes NCI put in place during the pandemic. Telehealth was rated most useful (avg. 4.6/5), followed by shipping oral IND agents to enrolled patients (4.5/5), remote informed consent (4.2/5), coordinating care with local providers (3.9/5), and remote auditing (3.7/5). Conclusions: The cancer trials community has an opportunity to learn from working through the challenges of COVID-19. NCI will seek to continue and expand on modifications to clinical trial processes that have the potential to improve operational efficiency, reduce cost, and help bring trials to more patients.