Background: Olfactory disorder is an early manifestation of Parkinson's disease (PD), likely to be associated with abnormalities of the dopaminergic neurons in the olfactory bulb (OB); however, the causes of olfactory disorder in PD are not entirely clear. Some studies showed that melatonin (MT) and androgens (mainly testosterone, T) might participate in the pathogenesis of PD. The research aimed to investigate effects of MT or T deficiency on OB dopaminergic neurons in rats.;-;Methods: One hundred and twenty normal male Wistar rats were randomly divided into the control, sham operation pinealectomy (PX), sham operation gonadectomy (GDX), PX, GDX, and PX + GDX groups. After 60 days, glial cell hyperplasia and neuronal apoptosis were examined with hematoxylin and eosin and the TUNEL method; the expression levels of tyrosine hydroxylase (TH), Bax, and Bcl-2 were measured using immunohistochemistry (IH) by the streptavidin peroxidase conjugated method. Comparison among multiple sets used analysis of variance and LSD method or Kruskal-Wallis test and Nemenyi method.;-;Results: There were no significant differences between the sham operation groups and the control group; thus, they were merged into Group A. There was no significant glial cell hyperplasia (P > 0.05) or change in shape in any of the groups after PX or GDX. The number of apoptotic cells in Groups A (1.41 +/- 0.56), PX (12.31 +/- 4.68), GDX (20.52 +/- 5.13), and PX + GDX (30.23 +/- 5.25) successively significantly increased (P < 0.05). The number of TH (+) cells in Groups A (42.62 +/- 5.63), PX (37.31 +/- 4.32), GDX (31.07 +/- 4.21), and PX + GDX (25.22 +/- 3.66) was successively significantly decreased (P < 0.05). The gray value of TH (+) cells and fibers in Groups A (98.51 +/- 10.36), PX (108.96 +/- 13.01), GDX (119.02 +/- 12.98), and PX + GDX (128.99 +/- 13.39) was successively significantly increased (P < 0.05). The results of Bax staining were as follows: Group A+, Group PX++, Group GDX++, and Group PX+ GDX+++, the results of Bcl-2 in all groups were +.;-;Conclusions: PX or GDX could lead to OB neurotoxicity in the following groups of rats in the following order: PX < GDX < PX + GDX. PX or GDX increased the ratio of Bax/Bcl-2. The effect of PX and GDX was equal, but both were less than that of PX + GDX. Neurotoxicity as a result of PX or GDX was not related to inflammation.