The aim of our study was to analyze the physical and histological behavior of a second generation collagen (GAX 65*) in the mini-pig bladder, comparing it to the commonly used substance for injection (GAX 35*). GAX 65 is a glutaraldehyde cross-linked bovine collagen with a collagen concentration of 65 mg./ml. This much higher collagen concentration could be achieved due to sophisticated manufacturing technology. In 18 mini-pigs 2 to 4, 1 ml. deposits of GAX 65 and 35, respectively, were implanted into the submucosal space of the surgically opened bladder. The animals were sacrificed at months 1, 2, 4 and 6. The elevation (height equals protrusion of the implant into the bladder) and volume of the implants were measured and calculated immediately after injection and at necropsy. Histology was performed. The GAX 65 implants had a lesser decrease of elevation and volume during the 6-month observation period. Histology showed that the invasion of endogenous fibroblasts into the GAX 35 and 65 implants, and the formation of porcine collagen types I and III were almost identical. Clinically GAX 65 has better elevation properties than GAX 35 although statistical analysis did not show any significant difference (p = 0.07 to 0.25). However, preservation of the volume of the GAX 65 implant was significantly superior (p < or = 0.05) compared to that of GAX 35. Therefore, GAX 65 will most probably prove to be an excellent substance for the endoscopic treatment of vesicoureteral reflux and incontinence in children.