Physical conditions of H II regions in M101 and the pregalactic helium abundance

Spectrophotometry in the 3400-7400 A range is presented for eight H II regions and the nucleus of M101. The He, N, O, Ne, S, and Ar abundances relative to H are derived. The O/H ratios are smaller than previously found by about 0.2 dex. We find negative gradients with galactocentric distance of the O/H, N/O, and He/H ratios. We do not find any gradients in the S/O, Ne/O, and Ar/O ratios. The pregalactic helium abundance by mass has been determined from NGC 2363, NGC 5471, and the SMC H II regions and amounts to 0.230 +/- 0.006 (1 σ).