  We present deep spectrophotometry of 18 HII regions in the nearby massive
spiral galaxies M101 and M31. We have obtained direct determinations of
electron temperature in all the nebulae. We detect the CII 4267 line in several
HII regions, permitting to derive the radial gradient of C/H in both galaxies.
We also determine the radial gradients of O/H, N/O, Ne/O, S/O, Cl/O and Ar/O
ratios. As in other spiral galaxies, the C/H gradients are steeper than those
of O/H producing negative slopes of the C/O gradient. The scatter of the
abundances of O with respect to the gradient fittings do not support the
presence of significant chemical inhomogeneities across the discs of the
galaxies, especially in the case of M101. We find trends in the S/O, Cl/O and
Ar/O ratios as a function of O/H in M101 that can be reduced using Te
indicators different from the standard ones for calculating some ionic
abundances. The distribution of the N/O ratio with respect to O/H is rather
flat in M31, similarly to previous findings for the MilkyWay. Using the disc
effective radius, Re, as a normalization parameter for comparing gradients, we
find that the latest estimates of Re for the Milky Way provide an excess of
metallicity in apparent contradiction with the mass-metallicity relation; a
value about two times larger might solve the problem. Finally, using different
abundance ratios diagrams we find that the enrichment timescales of C and N
result to be fairly similar despite their different nucleosynthetic origin.
