Objective To cladfy the dependence of neural nitric oxide synthase mRNA (nNOSmRNA) and endothelial nitric oxide synthase mRNA (eNOSmRNA) on androgens (testosterone [T] and dihydrotestosterone [DHT]). Methods 160 male Sprague Dawley (SD) rats were divided into Groups A (56 rats, 5 weeks old), B (50 rets,10 weeks old) and C (54 rats, 58 weeks old). Groups A, B and C were all subdivided respectively into five Subgroups. Subgroup 1: intact osntrels; Subgroup 2: castrated; Subgroup 3: castrated with testosterone ubdecanoate 25 mg/kg·mon-1, intramuscular injection, Subgroup 4: castrated with testosterone undecanoate 50 mg/kg·mon-1, intramuscular injection and Subgroup 5: treated with finaeteride 4.5 mg/kg·day-1, orally. Four and ten weeks after treatments described above, one half of the rats were killed. Serum samples were token for measurements of T, free testosterone (FT) and DHT by raclioimmunoassay. Penile samples were treated with liquid nitrogen and then stored at-80℃. nNOSmRNA and eNOSmRNA were detected by semiquantitative reveres-transcription polymerase chain reaction (RT-PCR) and Dot blot. Resulte There was no significant difference between Subgroup 1 and Subgroup 2 or Subgroup 5 in all Groups A, B and C. The expression of penile eNOSmRNA of Group A was significantly increased (4 weeks model) (P＜0.05) or increased (10 weeks model) (P＞0.05) in Subgroup 2 or 5 compared with those in Subgroup 1.There wes no significant difference between Subgroup 1 and Subgroup 2 or Subgroup 5 of Group B in 4 weeks model (P＞0.05). There was an elevation when animals were castrated or treated with finasteride in the 10 weeks model.The expreseion of penile eNOSmRNA of Group C was significantly increased (10 weeks model) (P＜0.05) or increased (4 weeks model) in Subgroup 2 compared with those in Subgroup 1.The production of eNOSmRNA in Subgroup 5 was also increased (including 4- and 10-week models). When T was supplied for castration, the penile eNOSmRNA was desreased to some extent; the greater the dose of T given, the lower penile eNOSmRNA was observed. Conclusions The expression of eNOSmRNA in SD rat penile tissue increases while T or DHT diminishes. Sometimes androgens medaulate penile eNOSmRNA in opposite directions. There is no srrelation between the expression of nNOSmRNA and androgens (including T and DHT) . Androgens give rise to penile erection probably not via the NOS pathway.