We have performed two-dimensional spectroscopy of the Seyfert galaxies NGC 5728 and 4151 in the 4500-7500A spectral range. The data (especially the [OIII] lambdalambda4959, 5007 lines), obtained using a new optical fibre system, complement previous observational work carried out by means of this technique. The two gaseous components found in NGC 5728 seem to be ionized by the same source, although they have different kinematics. They coexist (projected) in a strip-shaped zone aligned in the NE-SW direction. We propose that one of the components, probably related to outflow, energies at the north-west edge of the obscuring material which hides the nucleus and gives rise to the biconical structure of ionization observed in NGC 5728. The cones of ionizing radiation seem to be tilted with respect to the galaxy's plane. The velocity field of NGC 4151 derived from the ionized gas is complex, showing two receding poles. Hence the location of the kinematical centre is not straight-forward and the active nucleus could be off-centre. The maxima of the line-intensity and several continuum maps are in positional agreement within an uncertainty of +/-0.13 arcsec, however, in contrast with previous results. In addition, from the elliptical fitting of the surface brightness distribution in the I band, it may be inferred that the optical nucleus lies at the centre of a remarkably regular and spherical galactic bulge. This seems to establish the identification of the optical nucleus with the galaxy's mass centroid. One result common to these galaxies is that similar kinematical properties are inferred from the [OIII] and Hα narrow emission lines. This contrasts with the differences found in other Seyfert galaxies like NGC 7469 or 3227 where two ionization sources (starburst-like and AGN-like) associated with different kinematical systems seem to coexist.