Magnetic field (MF) has had many therapeutic usages in traditional medicine. Today, it is used to accelerate the recovery period after surgical operations by some medical centers and in some horse-training farms in the world. There are some scientific reports on the alternative effect of MF on nervous system function, convulsion, pain, neurons, embryonic cells and immune system cells. For this reason, the effect of constant MF on formalin-induced pain was investigated in the present project. For this purpose, two magnets (80 and 120 militesla (mt) at 15 centimeter distance from each other) were used. It was 4 mt in the middle of 30 cm distance between two magnets when they were opposed to each other by different poles. These three MF powers were used to perform the tests. Tests were designed in two categories; i.e. acute and chronic. In the acute experiment, formalin test (FT) was performed in three groups (n = 6) during exposure MF power. In the chronic case, groups (n = 9) were exposed to MF for two hours a day for 10 days and then FT was performed. Results of the acute tests showed additive effect of MF on pain sensation, as MF with 80 mt and 120 mt powers had 15.5 % and 19.5% additive effects (P<0.01) in the first phase, and 3.7% and 14.9% additive effects (P<0.05) in the second phase of FT respectively. Results of the chronic tests showed two opposite effects in the first and second phases of FT. In this respect, MF with 4, 80, and 120 mt power had 8% (P<0.05), 1.5%, and 3.5% additive effects in the first phase, and 14.9%, 16.7% and 31% attenuating effect (P<0.01) in the second phase respectively. Some reports have shown MF acutely evokes calcium channels and increases intracellular concentration of calcium and so affects the cell excitability. It may also increase the excitability of pain receptors and its pathway in the central nervous system (CNS), so it is a probable reason for acute additive effect of MF. It has also indicated that chronic MF increases release of serotonin and enkephalins. Also, the inhibitory effect of long-term high intracellualr calcium concentration on the cell activity has been proved. These two mechanisms may be responsible for chronic attenuation of pain sensation at the second phase of FT.