Nuclear relaxation rates of13C and14N have been measured as a function of temperature for neat liquid pyrimidine and pyridazine. Nuclear Overhauser enhancements, also measured as a function of temperature, were used to separate dipolar and spin rotational contributions to13C relaxation.13CH dipolar relaxation rates were combined with quadrupolar14N relaxation rates to determine rotational correlation times for motion about each principal molecular axis. The accuracy of the results is limited primarily by uncertainty in liquid-state values of the14N quadrupole coupling parameters. The temperature and viscosity dependence of the rotational correlation times is compared with those of other six-membered aromatic molecules, and is discussed in terms of the relative influence of molecular shape, attractive forces of dipolar origin and self association.2H and14N relaxation rates and the2H quadrupole coupling constant for pyrazine are also reported here, but the motion of this molecule could not be analysed.