We have recently developed an immunocytochemical technique for staining estrogen receptors in cell nuclei of some cells in the guinea pig brain. With optimization of this technique, we have now been successful in staining estrogen receptor-immunoreactivity in processes of many neurons in the guinea pig brain that also contain estrogen receptor-immunoreactive cell nuclei. While reaction product is visible in cell nuclei under a wide variety of conditions, neuronal processes are darkly immunostained only with modifications of the procedure optimized for maximum sensitivity, for example, the multiple-bridge immunocytochemical procedure. Omission of Triton X-100 and/or dimethylsulfoxide, usually used to increase penetration of the antibodies, had no effect on immunostaining in processes or cell nuclei, and immunostaining was apparent in both vibratome-cut and freezing microtome-cut sections. Injection of estradiol, but not progesterone, caused the total loss of cytoplasmic estrogen receptor-immunostaining, consistent with the idea that the cytoplasmic immunostaining, like the cell nuclear immunostaining is due to the presence of estrogen receptors. In all neuroanatomical regions containing estrogen receptor-immunoreactive cell nuclei, associated processes of some, but not all cells were also immunostained. However, in certain areas, such as the midbrain central gray and the preoptic area, cytoplasmic staining was particularly dark. The cellular characteristics that result in immunostaining in some estrogen receptor-immunoreactive cells, and not in others is under investigation.