This study was designed to quantify the microglial response following an injection of N-methyl-D-aspartate (NMDA) into the sensorimotor cortex of 6-day-old rats. After survival times ranging from 10 h to 28 days, cryostat sections were processed for the demonstration of microglial cells by means of tomato lectin histochemistry. The injection of NMDA caused an extensive primary lesion involving the neocortex, the rostral hippocampus, and rostral thalamus. In addition, secondary retrograde/anterograde degeneration was also observed in the ventrobasal (VB) complex of the thalamus. Microglial reactivity was already present at 10 h postlesion and restricted to areas of neuronal degeneration. Quantitative analysis was performed on digitized images using NIH Image software and a Macintosh computer. The method is based on densitometric ratios, referred to as the "reactivity grade," between the ipsilateral lesion side and the contralateral control side. Measurements were made to determine a possible increase in the number of microglial cells as well as an increase in lectin binding. The analysis showed that microglial reactivity in areas of primary degeneration peaked at 3 days postlesion, when it was significantly (P < 0.01) higher in comparison to saline-injected litter mates. Microglial response in the cerebral neocortex, showing the highest reactivity grade, as well as in other areas of primary degeneration, returned to control levels by Day 7. Microglial response in the VB complex also peaked at Day 3 (P < 0.05) but maintained this level of reactivity until 7 days postlesion (P < 0.01).