We obtain the Na I lambda 8183,8195 absorption line radial velocity curves for the polars ST LMi and MR Ser, from which we find the semi-amplitudes to be Kabs=329 +/- 9 km/s and Kabs=289 =/- 9 km/s respectively. We find that for both systems the effects on the Na I absorption lines due to X-rays heating the inner face of the secondary are negligible, and so the values obtained for Kabs can be taken as the true semi-amplitude of the secondary star. We then determine the projected rotational velocities, Vrot sin i, to be 104 =/- 9 km/s and 66 + /- 13 km/s for ST LMi and MR Ser respectively which enables their mass ratios to be calculated. For ST LMi and MR Ser we find the mass ratio to be 0.22 +/- 0.04 and 0.10 +/- 0.05 respectively; values which are significantly different only at the 94 percent level. Using the value for the orbital inclination derived from polarimetric measurements, we determine the mass, and the orbital and rotational velocities of the secondary stars. These are significantly different at less than the 90 percent level. However, if the limb darkening is the same in both objects, these quantities are significantly different at the 96 percent level. We present Doppler maps of the Na I absorption and Ca II emission in ST LMi and MR Ser. In both systems the Na I absorption covers the secondary star. In ST LMi the Doppler image of the Ca II emission shows that it originates at the inner face of the secondary star. In MR Ser, however, the emission lies close to but not on the secondary star. We show that 'spike' in the orbital period distribution of polars is a significant feature. although the discovery of only one more system with a period outside the 'spike' would decrease its significance below a 99 percent confidence level. We conclude that, even if the limb darkening coefficients for the secondary stars in ST LMi and MR Ser are the same, we cannot rule out the two systems having identical parameters. Therefore our observations are compatible with the theory explaining the 'spike' in the period distribution of the AM EIers.