Erectile dysfunction in the aging male results in part from the loss of compliance of the corpora cavernosal smooth muscle due to the progressive replacement of smooth muscle cells by collagen fibers. We have examined the hypothesis that a spontaneous local induction of inducible nitric oxide synthase (iNOS) expression and the subsequent peroxynitrite formation occurs in the penis during aging and that this process is accompanied by a stimulation of smooth muscle apoptosis and collagen deposition. The penile shaft and crura were excised from young (3–5 mo old) and old (24–30 mo old) rats, with or without perfusion with 4% formalin. Fresh tissue was used for iNOS and proteasome 2C mRNA determinations by reverse transcription polymerase chain reaction assay, ubiquitin mRNA by Northern blot, and iNOS protein by Western blot. Penile sections from perfused animals were embedded in paraffin and immunostained with antibodies against iNOS and nitrotyrosine, submitted to the TUNEL assay for apoptosis, or stained for collagen, followed by image analysis quantitation. A 4.1-fold increase in iNOS mRNA was observed in the old versus young tissues, paralleled by a 4.9-fold increase in iNOS protein. The proteolysis marker, ubiquitin, was increased 1.9-fold, whereas a related gene, proteasome 2c, was not significantly affected. iNOS immunostaining was increased 3.6-fold in the penile smooth muscle of the old rats as compared with the young rats. The peroxynitrite indicator nitrotyrosine was increased by 1.6-fold, accompanied by a 3.6-fold increase in apoptotic cells and a 2.0-fold increase in collagen fibers in the old penis. In conclusion, aging in the penis is accompanied by an induction of iNOS and peroxynitrite formation that may lead to the observed increase in apoptosis and proteolysis and may counteract a higher rate of collagen deposition in the old penis.