The objectives were to describe the associations between splenic reservoir mobilization and pharmacological changes in blood pressure (BP), the normal daily variability of BP, and the variation in BP between measurements obtained at a veterinary hospital and at home. A group of horses from a research herd (research group) and a group of healthy client-owned horses presented to a hospital (hospital group) were studied. Invasive BP and splenic volume were measured in the research group while hyper- and hypotension were induced by phenylephrine and acepromazine, respectively. Noninvasive blood pressure (NIBP) and ultrasonographically estimated splenic volume (by 2 observers) were measured on three consecutive days in research group horses. Noninvasive BP was measured in the hospital and the home environment in the hospital group at three time points. The change in splenic volume was highly correlated with the change in systolic BP after phenylephrine administration (r = −0.97, P ≤ .001), and there was no significant change in splenic volume or correlation with changes in BP after acepromazine administration. Noninvasive BP had low daily variability (coefficient of variation [CV] = 9%–11%), and splenic volume had high daily variability (CV = 30%–32%) but was reproducible between observers (intraclass correlation coefficient = 0.70). Horses in the hospital group repeatedly had higher systolic NIBP in the hospital (140.0 ± 14.0) than at home (133.8 ± 14.8, P = .01). Variation in BP after administration of phenylephrine, but not after administration of acepromazine, is related to the change in splenic volume. BP does not change significantly in consecutive days; however, splenic volume does and is reproducible between observers. Blood pressure in horses is higher in hospital than that in home.