We analyze time sequences of photospheric spectral lines in the penumbra of two sunspots. Two spectral lines sensitive to velocity fluctuations at two different heights in the atmosphere (Ti I 8396.9A and Fe II 7224.5A) are used. The observations were taken using the Gregory-Coude Telescope (GCT) at the Observatorio del Teide (Tenerife). Velocity fluctuations were determined by measuring the shifts of the line cores. The oscillatory power was calculated at three frequency bands: low frequency (1.5-2.5mHz), and five (2.8-3.8mHz) and three minute bands (5.1-6.1mHz). The main results are: -For the first time penumbral oscillations are clearly detected in the deep layers of the photosphere (Fe II line). -Significant variations are found between the inner, medium and outer parts of the penumbra. Based on a geometrical model for the average inclination of the magnetic field lines and assuming that these are co-spatial with the filaments, we conclude that for the lower photospheric layers (Fe II line) the velocity perturbations, at the inner and middle penumbra, are aligned with magnetic field lines. This is clearly not the case for the outer penumbra. However, the Ti line, formed in the upper layers, exhibits a more irregular behaviour. Hence, the oscillatory behaviour of the penumbral upper layers does not seem to be related to the filamentary topology visible in the continuum.