We aimed to investigate the effects of continuous exposure to hypomagnetic field on neurotransmitter, hormone content and blood routine parameters in juvenile (three weeks old) and adult (eight weeks old) male C57BL/6 mice (Mus musculus domesticus). The experimental group was exposed to the hypomagnetic field at density of 1.1 ± 0.5 μT, while the control group was exposed to the geomagnetic field at density of 53.3 ± 0.2 μT. A total of 84 and 72 male juvenile and adult mice were randomly divided into the experimental group and the control group, 42 and 36 in each group, respectively. Juvenile mice were exposed to the two kinds of magnetic field for 1 to 5 weeks while adult mice for 1, 3, 6, 9, and 12 weeks. The contents of neurotransmitters and hormones including gamma-aminobutyric acid (GABA), serotonin (5-HT), dopamine (DA), acetylcholine (Ach), noradrenaline (NA), thyroxine (T4) and growth hormone (GH) in the brain tissue or serum samples were measured using LC-MS and ELISA. Blood routine parameters were also measured in Peking University Health Science Center. We compared the results of the experimental and control groups with independent sample t-test. The results showed that the contents of 5-HT and NA in the brain tissues of adult mice exposed to hypomagnetic field decreased significantly after 1-week exposure (P < 0.05) (Fig. 1), and the contents of 5-HT in serum decreased significantly after 3-week exposure (P < 0.01). The contents of T4 decreased significantly at the 3rd, 6th, 12th week (P < 0.05) (Fig. 2). The content of DA in brain tissues of juvenile mice decreased at the 5th week (P < 0.05) (Fig. 1). The content of T4 in serum decreased significantly at the 2nd and 5th week (P < 0.05) while NA decreased significantly at the 4th week (P < 0.01) (Fig. 2). Blood routine examination showed that white blood cells (WBC) increased significantly at the 1st week for adult and 2nd week for juvenile mice, respectively (P < 0.01). Other parameters had no significant differences (Fig. 3 and appendix 1). This study shows that the parameters responding to the exposure to hypomagnetic field for juvenile and adult mice are not completely consistent, excepting that T4 reduces significantly in both groups. The results indicate that T4, 5-HT, NA, DA and WBC may be physiological parameters sensitive to the hypomagnetic-field exposure, which should be paid special attention to in future studies.