Rapid immobilization after acute spinal cord injury (SCI) leads to increased bone resorption, net calcium efflux from the bone, hypercalciuria, depressed parathormone (PTH) and increased calcitonin release. However, the effects, if any, of long-standing SCI on calcium regulatory system is not well understood. We measured plasma concentrations of 25 hydroxy (OH) vitamin D, 1,25(OH)2 vitamin D (calcitriol), intact PTH molecule, calcitonin, ionized calcium [Ca++] and phosphorus in 40 clinically stable men with long-standing SCI of 3-year to 50-year duration (22 persons with paraplegia and 18 persons with quadriplegia). The results were compared with those obtained in 14 able-bodied control men. Plasma PTH concentration in the SCI group was significantly lower than that found in the able-bodied controls despite virtually identical concentrations of ionized calcium. Likewise, plasma calcitriol concentration in the SCI group was significantly lower than the value found in the able-bodied control group and lower in persons with quadriplegia than in those with paraplegia. In contrast, plasma calcitonin concentration in the quadriplegic group was significantly higher than that in persons with paraplegia and insignificantly higher than that in the control group. No significant difference was noted in serum ionized calcium between the study groups. PTH and calcitriol levels were positively related to one another (r = 0.35, p < .01) and negatively related to the level of injury (r = -0.43, p < .002 and r = -0.54, p < .001, respectively). In conclusion, long-standing SCI is associated with significant depression of calcitriol and PTH concentrations despite normal ionized calcium concentration.(ABSTRACT TRUNCATED AT 250 WORDS)