The submandibular glands of rats raised at room temperature show daily and seasonal variations. To quantify these variations, the submandibular glands of male Wistar rats were collected at 4 h intervals over a 24h period in the summer and winter and processed for histological analysis. Gland weight decreased 37% (P<0.01) in the winter and 13% in the summer. The volume of the acinar compartment decreased 73% (P<0.01) between midnight and 4 a.m. in the winter and 38% (P<0.01) between 8 p.m. and midnight in the summer. In contrast, the volume of the stroma increased about 164% (P<0.01) between midnight and 4 a.m. in the winter and by 10% (P<0.01) between 8 p.m. and midnight in the summer. The volume of the duct system generally remained constant, although in the winter the convoluted granular tubules diminished 44% (P<0.01) between midnight and 4 a.m. Although the volume of the intercalated ducts remained constant over 24 h in both seasons, there was a 49% decrease in winter compared to summer (P<0.01). These results indicate that the large changes in gland weight were caused mainly by variation in the volume of the acini, convoluted granular tubules and stroma. The morphological variations were much more pronounced in winter than in summer and this affected the daily saliva production.