Noncystic periventricular leukomalacia (PVL) in premature infants is becoming a predominant lesion form. However, detection of the gray matter (GM) lesions involved in this disease, which closely relate to the later cognitive and behavioral deficits, is challenging because of their subtle and transient nature observed by conventional MRI. This study evaluated manganese-enhanced MRI (MEMRI) for detecting such GM lesions in 7-day-old rats with mild hypoxic-ischemic (H-I) insult, a characteristic model of noncystic PVL. Group 1 (n=6) and Group 2 (n=8) were administered intraperitoneally with MnCl(2) at hour 3 and day 7 after H-I insult, respectively. Control Group (n=6) received no MnCl(2). T1-, T2- and diffusion-weighted imaging (T1WI, T2WI and DWI, respectively) was performed. Animals were sacrificed for H&E staining, and immunohistochemical staining for glutamine synthetase (GS) and Mn-superoxide dismutase (Mn-SOD), which are two Mn-binding enzymes against glutamate toxicity and oxidative stress respectively in neurodegeneration. In Control Group, MRI appearance of H-I lesions normalized by day 7 after H-I insult. In Group 1, MEMRI provided the enhanced and prolonged GM lesion detection from day 3 up to day 21. In Group 2, similar Mn enhancement was observed, enabling day 8 detection of GM lesions that were invisible before Mn injection at day 7. These in vivo Mn-induced GM lesion enhancements were found to correlate with increased immunoactivities of GS and Mn-SOD. These findings suggest the potential utility of MEMRI in detecting the GM lesions that are otherwise undetectable using the conventional MRI techniques in late phase of mild H-I injury.