['To investigate the effects of growth hormone (GH) on the erectile function and the number of neuronal nitric oxide synthase (nNOS) -containing nerve fibers in the penis of aged rats.', 'Twenty-four aged male SD rats (18 months) were randomized into 2 groups: GH intervention group and control group. After four and eight weeks, a half of each group were selected and tested for erectile function after apomorphine (APO) injection and then sacrificed for the detection of nNOS-containing nerve fibers in the penis by streptavidin-peroxidase conjugated method (SP method).', 'After four weeks, the erectile function and the number of nNOS-containing nerve fibers showed no significant difference between the GH intervention group and the control group (P > 0.05). After eight weeks, the erection frequency was significantly different (P < 0.05) between the two groups, while the erection rate was not. The number of nNOS-containing nerve fibers in the GH intervention group was significantly larger than that in the control group (P < 0.01).'] Does [ Growth hormone enhance regeneration of nNOS-containing nerve fibers in the aged rat penis ]?