To test for the effects of psilocybin on depressive-like behavior, we used the FST as our primary outcome measure. The FST is a behavioral despair paradigm often used to measure depressive-like behavior and to screen for antidepressant-like effects in rats. (59) The FST interprets immobility, a passive coping strategy, as a depressive-like behavior, and reductions in immobility as evidence of antidepressant-like effect. (57,59) The FST measures coping strategies that are not solely mediated by neurocircuits known to be dysfunctional in the human demographic and thus lacks construct validity. However, it remains the most reliable measure of passive coping strategy, a depressive-like behavior, in both male and female rats, (60,61) as measures of anhedonia like the sucrose preference test have been shown to be inconsistent in females (60,61) and negatively correlated to behavioral despair independent of chronic stress paradigms. (62) The FST can be used to screen for antidepressant-like action of drugs given to otherwise healthy rats, (57,60) for depressive-like behaviors in animal models of depression, (63,64) or for antidepressant-like action of drugs given to animals modeling depression. (63−65) Our use of the term “antidepressant-like” when referring to the effects of psilocybin here are in reference to the published effects of antidepressants typically used to treat humans on rat behaviors, and not to the effects of antidepressants on human behaviors. As the FST measures active behaviors, overall locomotor activity also must be assessed immediately prior to the FST to control for nonspecific sedative or stimulant effects that might otherwise lead to false positive or negative results. (57) Increased immobility or activity in the FST is only meaningful if overall locomotor activity is unchanged, which we found to be the case in each of our experiments. A 15 min pre-exposure to the paradigm is required to elicit measurable depressive-like behavior in otherwise healthy rats. It had been widely assumed that each subsequent/additional exposure to an FST would increase immobility. (66) This assumption was shown to be false in a 2011 study that used a single 15 min pre-exposure and weekly FST. (66) Multiple experiments were performed during our study, using both repeat measures and interval testing. While there is evidence of differences in FST behavior between distinct phases of a rat’s life, prepubertal (<4 weeks), adult (2–18 months), and aged rats (18 months), (56) all tests in this study occurred during the same phase, young adulthood (9–15 weeks), (67) during which time FST behaviors are known to be consistent. Thus, we chose to reduce animal resources in studies with repeat measures by comparing our interval-tested treatment groups with the first FST of the repeatedly tested saline groups. Although our intent was not to confirm the validity of repeat measures FST, our results are consistent with those of Mezadri et al., (66) in that we found repeated exposure to the FST did not increase or decrease immobility in control rats. However, antidepressant-like effects in animals given psilocybin and repeatedly exposed to the FST were not as robust as in those given the same drug, but only exposed to a single FST. We address this finding in the Results and Discussion section.