Eight members of the He II resonance series have been studied photographically at electron densities around 1.4*10¹⁷ cm ^-3 in a reproducible Z-pinch plasma. The results show that small shifts to shorter wavelength occur although the magnitudes are smaller than 0.03 AA even for the 9s-1p transition (for which the largest shift is expected theoretically). The measured shifts are compared with several theories proposed previously.