Throughout the world, including the United States, men have worse outcomes from COVID-19 than women. SARS-CoV-2, the causative virus of the COVID-19 pandemic, uses angiotensin-converting-enzyme-2 (ACE2) to gain cellular entry. ACE2 is a member of the renin-angiotensin-system (RAS) and plays an important role in counteracting the harmful effects mediated by the angiotensin-type-1-receptor. Therefore, we conducted Ovid MEDLINE and Embase database searches of basic science studies investigating the impact of the biological variable of sex on ACE2 expression and regulation from 2000, the year ACE2 was discovered, through December 31, 2020. Out of 2131 publications, we identified 853 original research articles on ACE2 conducted in primary cells, tissues and/or whole mammals excluding humans. The majority (68.7%) of these studies that cited the sex of the animal were conducted in males, while 11.2% were conducted solely in females; 9.26% compared ACE2 between the sexes, while 10.8% did not report the sex of the animals used. General findings are that sex-differences are tissue-specific and when present, are dependent-upon gonadal state. Renal, cardiac and adipose ACE2 is increased in both sexes under experimental conditions that model co-morbidities associated with worse COVID-19 outcomes including hypertension, obesity, and renal and cardiovascular diseases; however, ACE2 protein was generally higher in the males. Studies in Ace2- knockout-mice indicate ACE2 plays a greater role protecting the female from developing hypertension than the male. Studying the biological variable of sex in ACE2 research provides an opportunity for discovery in conditions involving RAS-dysfunction and will shed light on sex-differences in COVID-19-severity.