Direct measurement of neuropeptides in the hypothalamus is essential for neuroendocrine studies. However, the small quantities of peptides released at their neuroterminals and relatively large molecular sizes make these measurements difficult. We have evaluated microdialysis probes with two membrane materials (polycarbonate and polyarylethersulfone, both: molecular cut off 20,000 Da) in vitro, and adapted the method for in vivo hypothalamic sample collection in non-human primates. The results of in vitro experiments showed that the polyarylethersulfone membrane yielded a several fold higher recovery rate than the polycarbonate membrane. In in vivo experiments, a guide cannula with stylet was inserted into the medial basal hypothalamus through the permanently implanted cranial pedestal under light sedation. The stylet was replaced by a microdialysis probe and artificial CSF was infused. The results indicated that the neuropeptide luteinizing hormone-releasing hormone was readily measurable in dialysates collected at 10 min-intervals, and responded to neuroactive substances applied through the probe. The animals were fully conscious except for the initial hour of sampling. After the experiment the animal was returned to the home cage, and later similarly examined during several additional experiments. Therefore, the microdialysis method described here is a highly useful tool for neuroendocrine studies in non-human primates.