Morphometry was performed using bright-field microscopy with a color video camera (JVC, Yokohama, Japan), video and computer monitors, and a computer. Stereological procedures that provide unbiased estimates of cell numbers were used.19- 22 Neurozoom software (Human Brain Project, La Jolla Calif) enabled digital superposition of the sampling grids on the tissue. Neuronal density and layer surface area measurements were performed on immunostained and cresyl stained sections, respectively. The 6 layers of the LGN were easily identified on stained sections. The layers were identified as layer 1 to layer 6 from ventral to dorsal. Ventral layers 1 and 2 are magnocellular layers, while the remaining dorsal layers 3 through 6 are parvocellular layers. Layers 1, 4, and 6 of the left LGN are connected to the glaucomatous right eye, while layers 2, 3, and 5 are connected to the nonglaucomatous left eye. To determine whether neurons are lost in magnocellular and/or parvocellular LGN layers connected to a glaucomatous eye, neurons in the left LGN layers 1, 4, and 6 were counted, and the counts were compared with those from the left LGN layers 1, 4, and 6 in control monkeys. Retinal ganglion cells of the right nasal hemiretina and fovea project to the left LGN layers 1, 4, and 6, and compose approximately 50% of the right eye retinal ganglion cells.9 The difference in nerve fiber loss between the nasal and temporal quadrants of the right optic nerves was not statistically significant (P>.05) for the monkeys examined in this study; therefore, changes observed in left LGN layers 1, 4, and 6 are most representative for changes observed in target LGN neurons. Left LGN layers 1, 4, and 6 of monkeys with a normal visual system were used as controls rather than right LGN layers 1, 4, and 6 of the laser-treated monkeys, since significant decrease in cell size has been observed in the undeprived layers in some monocular experimental conditions.23 In addition, our preliminary results show that the neuronal number in the left LGN layers connected to the control fellow eye in laser-treated monkeys appears to be reduced compared with the neuronal number in the left LGN in control monkeys (unpublished data, 1999). Analysis was performed for magnocellular layer 1 and parvocellular layers 4 and 6, the latter sharing the same morphologic and functional properties.