Destruction of the medial preoptic area (mPOA) neurons of rat brain, induced by intracerebral injection of N-methyl D-aspartic acid (NMDA), has been studied by employing the non-invasive Magnetic Resonance Imaging (MRI) technique. Changes in the MRI images are compared and correlated with the functional changes after the mPOA lesion. The progress of the lesion at the injected site has been monitored (using MRI) from 15 min to 1 month after the stereotaxic microinfusion of NMDA (5 micrograms in 0.2 microliter). This study shows that the localised hyperintense (bright) area starts appearing at the mPOA from 3 h after NMDA injection, and the brightness increases progressively for about 2 days. The size and brightness of hyperintense area decrease thereafter. It has not been possible to locate the lesion site after 3 days, using MRI, except in one rat where a vacuole-like area was seen at the NMDA injected site on postmortem histological examination. The reduction in sleep after the mPOA lesion does not show any correlation with the changes in MRI, as it persists throughout the 3 weeks of recording. On the other hand, the initial drastic reduction in male sex behaviour and the increase in body temperature correlated to some extent with the increased brightness in MRI at the site of lesion. The size and location of the hyperintense area, observed during the first 2 days, match with the lesioned area which was histologically identified after 1 month of NMDA administration. Control administration of normal saline into the mPOA did not produce any alteration in the brightness of the MRI image and practically no loss of neurons at the injected site. Though some functional changes have correlation with the alteration in MRI, this cannot be used to interpret the changes in all the physiological parameters. This study also demonstrates that the disappearance of the brightness in MRI should not be taken to indicate a positive prognosis. Though the lesion could not be seen in MRI within 2 hours, its detection after 3-4 h (but within 3 days) after NMDA lesion would give very valuable information for long term studies.