... We reconsider the problem of the difference between the abundances of carbon and oxygen in galactic planetary nebulae (PN) derived from the intensities of the recombination and collisionally excited lines. ... The formulae for calculation of the nebular line intensities in presence of both temperature and density fluctuations are given. ... C and O abundances We have fitted the line intensities for more than 70 PNe including all of those which were considered by RS except NGC 2242, IC 418 and M1-14....