It is well established that l,25(OH)2D3 is a major regulator of calcium homeostasis. The steroid exerts its effects on its target organs by two mechanisms, a slow mechanism mediated by nuclear vitamin D receptors (nVDR), and a rapid mechanism mediated by the l,25(OH)2D3 membrane-associated rapid response steroid binding protein (l,25D3-MARRS bp). In this dissertation, the physiological relevance of membrane initiated steroid signaling was investigated by studying the correlation of age in male and female chickens with the magnitude of responses to l,25(OH)2D3 in duodena from 7-, 14-, 28-, and 58-week-old birds. Measurements of l,25(OH)2D3 (130 pM) responsiveness as a function of age showed an age-related decrease in intestinal Ca2+ transport for both male and female birds. Western analyses on isolated basal lateral membranes (BLM) indicated a decreased expression of 1,25D3-MARRS bp with increasing age in male chickens, while its expression increased with age in female birds. Saturation analyses of [3H]l,25(OH)2D3 binding to BLM revealed an allosteric interaction identified as cooperative binding for both male and female birds. The degree of positive cooperativity increased with increasing age. Both genders showed a significant increase in dissociation constant (Kd) with increasing age, indicating decreasing affinity. No age-related changes in the number of binding sites, Bmax, were observed in male chickens, while female birds showed an increase in Bmax with increasing age. Data obtained in parallel binding analyses of [3H]l,25(OH)2D3 to nuclear fraction nVDR in contrast, indicated an absence of cooperative binding and an absence of significant changes in Kd or Bmax with age for both male and female birds. Membrane-initiated signal transduction by l,25(OH)2D3 was assessed by determination of protein kinase C (PKC) and A (PKA) activities. Both male and female birds showed an age-related decrease in stimulation of PKC activity in response to l,25(OH)2D3, while PKA activity increased. In conclusion, this dissertation demonstrates that there is a decrease in l,25(OH)2D3-induced intestinal calcium uptake as a function of age in duodena of both male and female chickens, which can be correlated to a decreased affinity for l,25(OH)2D3, and a decreased PKC activity.