Comprehensive (p, rho, T) measurements on pure nitrogen have been carried out in the single-phase region (920 values) and along the entire (vapour + liquid) coexistence curve (159 values). The results reported in this first paper cover the homogeneous gas and liquid region in the temperature range from 66 K to 340 K at pressures up to 12 MPa. The total uncertainty of the measurements in density rho is <(0.9.10(-4).rho to 1.5.10(-4).rho) with the exception of the temperature range from 123 K to 140 K (T-c = 126.192 K) at densities between 150 kg.m(-3) and 450 kg.m(-3), where the total uncertainty in pressure p is <(0.8.10(-4).p to 3.0.10(-4).p). Values for the second and third virial coefficients have been determined from the measurements in the homogeneous gas region of nitrogen. The experimental results are compared with the results Of previous workers.