The corticotropin-releasing hormone (CRH) neurosecretory system in normal rats consists of two major subpopulations of parvicellular neurons in the hypothalamic paraventricular nucleus distinguished by the presence or absence of coexistent vasopressin precursor (pro-AVP)-derived peptides. These neurons project to the external zone of the median eminence, where the two subtypes of axons (CRH +/AVP + and CRH+/AVP-) were previously found to be approximately equal in number. The present study was undertaken 1) to determine whether the relative numbers of pro-AVP expressing and pro-AVP deficient perikarya in the paraventricular nucleus corresponded to what we previously found for the axons in the median eminence, 2) to map the two cell types throughout the entire paraventricular nucleus to determine whether significant differences existed in their distributions, and 3) to ascertain whether or not the pro-AVP deficient subpopulation expressed pro-AVP after adrenalectomy. Postembedding electron microscopic immunocytochemistry on serial ultrathin sections was used to identify the peptide phenotypes of perikarya in the paraventricular nucleus in normal rats and 7 days after adrenalectomy with and without colchicine treatment. The peptide phenotypes of neuronal perikarya in the paraventricular nucleus were identified by using antibodies to CRH, AVP, neurophysin (NP), the C-terminal glycopeptide of pro-AVP (GP), and oxytocin-associated neurophysin (NPOT). Groups of three serial coronal ultrathin sections were analyzed at 200-micron intervals throughout the entire rostrocaudal extent of the paraventricular nucleus. The sections in each group were stained for CRH, a pro-AVP-derived peptide (AVP, NP, or GP), and NPOT, respectively. Parvicellular CRH neurons were defined as CRH-positive cells, approximately 10 micron in diameter, that did not contain detectable NPOT. Pro-AVP expressing cells were defined as staining positively for AVP, GP, or NP and negatively for NPOT. Approximately equal numbers of pro-AVP expressing ("NPAVP+") and pro-AVP deficient ("NPAVP-") parvicellular CRH neurons were found within the paraventricular nucleus of colchicine-treated normal rats, and the two subtypes were distributed differently within the paraventricular nucleus. Although the pro-AVP expressing CRH cells stained intensely for NP and GP, staining for AVP was quite variable and difficult to quantify in colchicine-treated normal animals.(ABSTRACT TRUNCATED AT 400 WORDS)