Fournier S, Joseph V, Kinkead R. Influence of juvenile housing conditions on the ventilatory, thermoregulatory, and endocrine responses to hypoxia of adult male rats. J Appl Physiol 111: 516 -523, 2011. First published May 19, 2011 doi:10.1152/japplphysiol.00370.2011.-"Extreme" housing conditions, such as isolation (single housing) or crowding, are stressful for rats, and their deleterious impact on behavior is well documented. To determine whether more subtle variations in housing can affect animal physiology, the present study tested the hypothesis that the hypoxic ventilatory response (HVR) of adult male rats housed in pairs during the juvenile period (postnatal day 21 to adulthood) does not differ from that of animals housed in triads. Because neonatal stress augments the neuroendocrine responsiveness to stress and HVR, experiments were performed both on "control" (undisturbed) animals and rats subjected to neonatal maternal separation (NMS; 3 h/day, postnatal days 3-12). At adulthood, ventilatory activity was measured by whole body plethysmography under normoxic and hypoxic conditions (inspired fraction of O2 ϭ 0.12; 20 min). The ventilatory and body temperature responses to hypoxia of rats raised in triads were less than those of rats housed in pairs. For the HVR, however, the attenuation induced by triad housing was more important in NMS rats. Triad housing decreased "basal" plasma corticosterone, but increased estradiol and testosterone levels. Much like the HVR, housing-related decrease in corticosterone level was greater in NMS than control rats. We conclude that modest changes in housing conditions (pairs vs. triads) during the juvenile period can influence basic homeostatic functions, such as temperature, endocrine, and respiratory regulation. Housing conditions can influence (even eliminate) the manifestations of respiratory plasticity subsequent to deleterious neonatal treatments. Differences in neuroendocrine function likely contribute to these effects. control of breathing; maternal separation; environment; chemoreflex; temperature regulation; hormone IN RATS, HOUSING CONDITIONS from weaning until adulthood have a significant impact on animal development and health. Extreme housing conditions, such as social isolation (single housing) or crowding (ϳ3.0 dm 3 /rat), are stressful to the juvenile animal and promote the emergence of anxiety, depression, and hypertension, and augment stress responsiveness at adulthood (7,36,50). By contrast, enrichment of the animal's environment with group housing, various objects, and larger cages facilitates natural social behaviors and improves animal health by making them more resilient to stressful conditions (31). In most animal care facilities, however, "standard" housing conditions (postweaning) consist of placing multiple animals within the same cage. The actual housing conditions are rarely documented, but a brief survey of the literature reveals important variations between laboratories: some report housing rats four to five per cage (36), whereas others ...