In view of the remarkable decrease of the relative heart weight and the relative blood volume in growing pigs, we investigated whether cardiac output (CO) and stroke volume (SV) of modern growing pigs are proportional to body mass (M), as predicted by allometric scaling laws: CO (or SV) = a.M(b), in which b is a multitude of 0.25 (quarter power scaling law). Specifically, we tested the hypothesis that CO scales with M to the power of 0.75 (CO = a.M(0.75)) and SV scales with M to the power of 1.00 (SV = a.M(1.0)) and investigated whether these relations persisted during increased cardiac stress. For this purpose, 2 groups of pigs (group 1 of 57 +/- 3 kg in Lelystad, and group 2 of 28 +/- 1 kg in Rotterdam) were chronically instrumented with a flow probe to measure CO and SV; instrumented pigs were studied at rest and during strenuous exercise (at approximately 85% of maximum heart rate). Analysis of both groups of pigs (analyzed separately or combined) under resting conditions demonstrated that the 95% confidence intervals of power-coefficient b for CO encompassed 0.75 and for SV encompassed 1.0. During exercise, similar results were obtained, except for SV in group 2, in which the 95% confidence limits remained below 1.0, which may have been due to the relatively small range of BW in group 2. These observations indicate that CO and SV of growing pigs with M less than 75 kg are still proportional to M, even during strenuous exercise, and that CO and SV scale with M according to the quarter power scaling laws. In conclusion, the concerns about disproportional growth and development of modern growing pigs with BW up to 75 kg were not confirmed by the present study.