Observed radio structures of jets in radio galaxies are compared with models based on beam and slingshot theories. Simple beam models predict a rapid decline of surface brightness B with distance r from the nucleus, Bproportionalr/sup -m/, where m>3. This contrasts with the almost uniform brightness out to 200 kpc in the jet of NGC 6251 and out to 10 kpc in the northern jet of 3C 31. On the other hand, the slingshot model gives a good agreement with the structure of both jets if the radio galaxies are embedded in an isothermal gas cloud with core radius of approx.80kpc (NGC 6251) and approx.8 kpc (3 C 31).