In early diastole, pressure is lower in the apex than in the base of the left ventricle. This early intraventricular pressure difference (IVPD) facilitates LV filling. We assessed how left ventricular (LV) diastolic IVPD and intraventricular pressure gradient (IVPG), defined as IVPD divided by length, scale to the heart size and other physiological variables. We studied 10 mice, 10 rats, 5 rabbits, 12 dogs and 21 humans by echocardiography. Color Doppler M mode data were post-processed to reconstruct IVPD and IVPG. Standardized LV filling time was calculated by dividing filling time by RR interval. The relationship between IVPD, IVPG, standardized LV filling time and LV end-diastolic volume (or mass) were fit to the general scaling equation Y =kM. LV mass varied from 0.049 to 194 g, while end-diastolic volume varied from 0.011 to 149 ml. values relating standardized LV filling time with LV mass and end-diastolic volume were 0.091±0.011 and 0.083±0.009, respectively (p<0.0001 versus 0 for both). values relating IVPD with LV mass and end-diastolic volume were similarly significant at 0.271±0.039 and 0.243±0.0361, respectively (p<0.0001 versus 0 for both). Finally, values relating IVPG with LV mass and end-diastolic volume were -0.118±0.013 and -0.104±0.011, respectively (p<0.0001 versus 0 for both). As a result, there was an inverse relationship between IVPG and standardized LV filling time (r=-0.65, p<0.001). We conclude that IVPD decrease while IVPG increase with decreasing animal size. High IVPG in small mammals may be an adaptive mechanism to short filling times.