The cytokine transforming growth factor beta1 (TGFB1) is implicated in male sexual function. Previous behavioral studies show that Tgfb1 null mutant mice mount and display limited intromission behavior with receptive females but are unable to complete successful copulation. The studies presented here explore the physiologic basis for sexual dysfunction in Tgfb1 null mutant males. Scanning electron microscopy revealed that the surface of the penis in Tgfb1 null mutant males was abnormally coated in superficial keratinized epithelial cells. There was a significant reduction in protrusion of penile spines through the superficial tissue in Tgfb1 null mutant mice; in some mice, the spines were almost completely embedded. Histologic analysis revealed reduced skin thickness in the penis of Tgfb1 null mutant males. Nerve fibers, endothelial cells, smooth muscle actin, macrophages, and neuronal and inducible nitric oxide synthase were present in similar abundance and location in Tgfb1 null mutant mice compared with wild-type controls; however, an increase in collagen I deposition was detected. Behavioral studies revealed that Tgfb1 null mutant males undergo spontaneous noncontact erections, albeit at a reduced rate compared with control mice, and engage in less frequent genital grooming activity. These studies suggest that Tgfb1 null mutation may adversely influence copulatory behavior through effects on both altered structural integrity of the penile skin and impaired tissue compliance leading to erectile dysfunction.