The potential of ISEE 1 spacecraft has been used to derive the plasma density in the magnetosphere and its environment. First, we show that using the equilibrium of currents flowing in and out of a spacecraft, we could numerically derive a relation between the spacecraft potential and the density of the surrounding plasma. After verifying that this relation was in good agreement with the measurement of the density made by other instruments during selected periods of time, we apply this relation to the spacecraft potential measured continuously from 1977 to 1984 on ISEE 1. An image of the plasma density in the magnetosphere and its environment is obtained as a result. All principal magnetospheric regions are clearly identified: the solar wind with a density around 5 cm−3, the magnetosheath with a density around 50 cm−3, the magnetosphere with a density around 1 cm−3, the plasma sheet with a density around 0.5 cm−3, and finally, the more tenuous tail lobes with a density below 0.1 cm−3. The plasma density was observed slightly higher on the dawnside than on the duskside of the magnetosphere. In addition, the magnetosheath was closer to the Earth on the dawnside than on the duskside. When the magnetic activity increased (recorded by the AE index), the dayside magnetosphere was compressed/eroded by about 1 to 2 RE while the plasmasphere/inner magnetosphere became quite irregular and expanded in the dawn-midnight and in the dusk-noon sectors. In addition, during high magnetic activity, the plasma sheet at YGSM = 0 was thicker and slightly denser than at low-activity. On the other hand, the flanks of the plasma sheet were thinner and slightly less dense during high-activity than during low-activity.