Regional pulmonary blood flow (PBF) in adult animals varies over space and time, following a fractal pattern. We hypothesized that PBF would follow a fractal pattern in young animals. Five, two-week old piglets were sedated and mechanically ventilated. After stabilization, fluorescent microspheres were injected via the right atrium at baseline and then again at 5, 20, 20.5, 40 and 60 min. The lungs were subsequently excised, dried, inflated, and cored into 0.12-cm pieces (mean n = 561 +/- 106 per animal) with the spatial coordinates recorded for each piece. Regional PBF was spatially and temporally heterogeneous with a spatial coefficient of variation of 43.3 +/- 7.9% and a temporal coefficient of variation of 14.3 +/- 0.4%. PBF followed a fractal pattern with a fractal dimension of 1.20 +/- 0.06 at 20 min, remaining stable throughout the experiment. PBF decreased with distance from the hilum but did not follow a lobar pattern. Temporal heterogeneity did not significantly increase with time but low flow regions demonstrated the greatest temporal variability throughout the study. Hence, PBF in young piglets was characterized both spatial and temporal heterogeneity.