The purpose of this study was to determine if splenic weight is correlated to changes in blood volume, haematocrit and haemoglobin concentration after epinephrine injection in horses. Ten mares, 5 racing type and 5 draft type, all over age 5 years were studied. Differences in plasma volume (PV), red cell volume (RCV), total blood volume (TBV) and venous haematocrit (PCV) were maximised by xylazine and epinephrine treatments to either sequester or mobilise the splenic RCV. Plasma volume was determined using an Evans Blue Dye (EBD) dilution technique, then used to calculate TBV and RCV. Horses were killed by captive bolt gun and exsanguination, and heart (HW) and spleen weights (SW) were recorded and expressed as a percentage of body weight (BW). Significant differences between breed types were found for changes in RCV and PCV between treatments. Analysis of variance found significant differences between breed types in HW/BW and SW/BW ratios. Regression analysis demonstrated a positive relationship between the change in RCV between treatments and SW/BW with significant differences (P < 0.01) between breed types in this relationship. There was no relationship between changes in RCV or PCV and HW/BW.