Pairs of evolutionary tracks have been calculated for stars of 3k, 5 and 7 solar masses; one track of each pair uses the Reeves rate for the N14(cc, ) F'8 reaction, in the other track this reaction is ignored. In the model of each pair with zero reaction rate, the convective envelope was 1-2 per cent deeper in mass fraction at the giant tip. At 5 and 7 solar masses, the models with deeper mixing had extended Cepheid loops; the less deeply-mixed models showed none. Accurate loop calculations at these models will require precise determination of the maximum depth of convective mixing. No loop appeared at 3k o Changing the envelope hydrogen distribution would not have produced a loop. This predicted absence of a loop at 3k o is consistent with observations.