(a) Cyclic P4 treatment increased sensitivity of male rats to low-dose ketamine (KET) treatment (****p < 0.0001, ***p < 0.001, **p < 0.01, *p < 0.05 vs. SAL) for up to 6 days following drug administration (Main Effects: Treatment/Day, ***p < 0.001; Hormone: ***p < 0.001). Data are expressed as mean ± SEM (n = 40). (b–e) Lower saline (SAL) baseline sucrose preference levels predicted a higher magnitude of positive response to KET in treatment-responsive P4-treated intact males (r2 = 0.9692, P < 0.0001). (f) Cyclic treatment with E2 alone or in combination with P4 significantly reduced overall body weight gain of intact male rats compared to Intact + OIL males (***P < 0.001), confirming effective hormone treatment. Data are expressed as mean ± SEM (n = 38).