To determine the effect of bovine growth hormone (bGH) on immune function in aging dogs, young (4 months), middle-aged (33-55 months), and old-aged (63-83 months) female beagles were studied. An age-associated decline in plasma thymulin concentration (as determined by the rosette-inhibition bioassay) was detected. Middle- and old-aged dogs were injected with either bGH or bovine serum albumin (control) for one month. Plasma thymulin concentration increased in every bGH-treated dog. In middle- but not old-aged dogs, bGH treatment resulted in rejuvenation of thymic morphological features as determined by stereotypical procedures. Thus, exogenous GH may be useful for restoration of some immune functions in aged individuals. To evaluate the effect of intermittent clonidine administration on GH release, thymic morphology, and immune function in aging dogs, 47-67 month old female beagles were given gelatin capsules (which were empty or contained clonidine pills) either every two days or every three days over a 30 day period. Release of GH was inconsistent and delayed perhaps due to delayed absorption of clonidine in pill form. Dogs receiving clonidine every two days had improved thymic morphology, but this was not significantly different from the other two groups. Clonidine treatment was associated with: significant enhancement of the primary antibody response to Brucella abortus but not to tetanus toxoid, and significant depression of serum blood urea nitrogen and glucose concentrations. Mitogenic response of lymphocytes and total serum IgG, IgM, and IgA were comparable to controls. The thymus is known to be part of the endocrine system and has hormone-producing epithelial cells. The age-involuted canine thymus has cysts lined by an epithelium of varying height, most often pseudostratified columnar with cilia, containing acidic and neutral mucins, some of which are sulfated; viable or necrotic cells are also present in the lumina. Immunocytochemical techniques revealed that some cyst epithelial cells contain thymosin alpha 1 or thymulin. Scanning electron microscopy revealed a mucous membrane appearance. Ultrastructurally, the apical region of epithelial cells contained numerous elongated mitochondria, smooth endoplasmic reticulum, well developed Golgi and membrane bound, electron dense vesicles with eccentric, dense cores. These cysts appear to be involved in secretion or absorption and degradation.