The effects of uniaxial-biaxial phase transition on the spectral densities measured from the deuteron Zeeman (T-1Z) and quadrupolar (T-1Q) spin-lattice relaxation times are examined in two partially deuterated samples of 4-n-pentyloxybenzylidene-4’-heptylaniline (50.7) : one deuterated at the methine site (50.7-d(1)) and one with the aniline ring deuterated (50.7 d(4)). The deuterated sites are chosen so that their relaxation behaviours are not complicated by internal chain dynamics. It is found that the spectral densities show discontinuities at the S-C - S-B phase transition, but show no discontinuity at the S-A - S-C and S-B - S-G phase transitions. Both the smectic C and G phases are biaxial. The results are discussed in terms of molecular reorientation and director fluctuations. Preliminary data for the sample director oriented at 90 degrees with the external magnetic field are reported in the B and G phases of 50.7-d(1).