Solar activity indices (coronal, chromospheric as well as photospheric) and cosmic ray neutron monitor rates (different cut-off rigidity) have been used to study 27-day variations in the years from 1957 to 2004. Daily data were employed for this purpose, analysed by the FFT and wavelet techniques. To work with a continuous data set for the cosmic rays (CR), the 'Composite Cosmic Ray' (CCR) set was first created from the observations carried out at different neutron monitor stations. The CCR frequency analysis shows significant 27-day variations in the intensity of CR, with its amplitude's values very sensitive to the sign of the quantity qA. The most significant 27-day variations of CR were found not to correlate with those of other solar indices.