We have previously reported anatomical and chemical changes in the cerebral cortex of rats living in an enriched, stimulating environment. The present study includes additional histological measures such as more extensive depth measures, differential cell counts, and cell size measurements.Celloidin-embedded, thionin-stained sections of the visual cortex from environmentally enriched rats and their impoverished littermates were measured. Two methods for calculating cortical depths were used: one, with an ocular micrometer taking eight measures lateral to the elevation in the corpus callosum on each hemisphere; two, on enlarged photographs taking 51 measurements 2 mm apart on each hemisphere. With the ocular micrometer method the cortical depth of the enriched brains was 6.4% greater than the depth of the impoverished cortex. O n the enlarged photographs the more medial depths in the enriched rats showed greater increases than the lateral depths.Overlapping photomicrographs were made of a medial cortical area 1.00 mm by 0.75 m m on each transverse section and were combined into one composite picture for each animal. Differential cell counts from the composite pictures were determined independently by two technicians. Results indicate a 14% ( p < 0.01) increase in glia among the enriched rats, No significant differences between enriched and impoverished groups were found in the perikarya and nuclear circumferences as measured with a planimeter from camera lucida drawings.We have previously reported that an enriched behavioral environment brings about measurable morphological changes in the rat brain (Diamond, Krech and Rosenzweig, '64; Bennett, Diamond, Krech and Rosenzweig, '64). In those experiments, depths and differential cell counts were taken on frozen, thionin-stained sections from the brains of rats living for 80 days in enriched or impoverished conditions. From these sections we found a 6.2% increase in the depth of the visual cortex of the rats from the enriched environment. The same sections were used for differential cell counts of the neurons and glia cells. The cells were counted in a series of microscopic fields proceeding from the pial surface of the cortex down to the underlying white matter. By this method, the number of neurons per field was about 17% fewer in the enriched rats while the number of glial cells per field was about 7% less; in other words, the J. COMP. NEUR., 128: 117-126.glial/neural ratio was 12% higher in the enriched rats than in the impoverished rats; a difference in this direction was found in 14 out of 18 pairs compared. All measurements and counts were done "blindly," i.e., the anatomist did not know whether she was working with a section from an enriched or an impoverished rat.The present study (experiments I11 and IV) can be seen as a replication and extension of the previous study to be referred to as experiments I and I1 (Diamond, Krech and Rosenzweig, '64). The new study includes more critical histological measures such as classifying types of glia and determini...