It is shown that cosmic ray and radio astronomical data are consistent with an open model of the galactic magnetic field, in which the lines of force run into intergalactic space and close around the local cluster of galaxies. The intergalactic magnetic field is associated with an ionized gas of density ∼10 −27 gm cm −3 . The halo of the Galaxy may be regarded as that part of the intergalactic gas which is under the gravitational control of the Galaxy. In this model the cosmic rays spend most of their time in intergalactic space. Their path-length is about 5 gm cm −2 , in contrast to a closed model, for which the path-length is about 30 gm cm −2 . In this respect the open model seems favoured by observation. The leakage rate out of the cluster is in approximate equilibrium with the production rate by novae and supernovae. Relativistic electrons also spend most of their time in intergalactic space, and the only important loss is again leakage out of the cluster. If the magnetic moment is a constant of the electrons' motion, the radio emissivity is proportional to H3.6 , which implies a magnetic field in spiral arms, disk, halo and intergalactic space of about 5 × 10 −6 , 2 × 10 −6 , 10 −6 , 5 × 10 −7 gauss respectively. Further implications of the model are (i) only 15 per cent of relativistic electrons are secondary (ii) the spectral index of the electrons may not be the same as that of cosmic ray protons (iii) the spectral index of the electrons should be the same for all parts of the cluster (except, perhaps, for localized sources).