Objectives. 1,25-dihydroxyvitamin D3 (calcitriol) inhibits prostate cancer growth in vitro and in vivo. We used a prostate microdialysis technique to better understand the intraprostatic pharmacokinetics of calcitriol, which in turn would facilitate planning for systemic calcitriol therapy in patients with prostate cancer. Methods. Male Sprague-Dawley rats were treated with 5 μg of calcitriol intravenously. Animals were either intact (group 1, n = 6) or castrated (group 2, n = 3). Prostate microdialysis was performed by perfusing Krebs solution through a 5-mm linear probe. Effluents were collected hourly from 0 to 20 hours or until death. Serum was collected at baseline and at the end of the experiment. Serum was also obtained from untreated rats at 0, 1, 2, 3, 4, 6, 8, 12, and 24 hours after intraperitoneal injection of calcitriol. Calcitriol levels were measured by radioimmunoassay. Results. The average baseline intraprostatic level of calcitriol in prostate dialysate in intact rats was 21.1 pg/mL (±7.5); it was 88 pg/mL (±98.4) after calcitriol administration. In castrated animals, the values were 16.6 pg/mL (±5.3) and 25.3 pg/mL (±10.7). Two peaks in intraprostatic calcitriol levels were observed after intravenous administration: at less than 6 hours after injection and at more than 13 hours after injection. The mean total calcitriol exposure (area under the concentration versus time curve) in the prostate was 297.6 (±159) pg/hr/mL (intact) and 272.7 (±123.6) pg/hr/mL (castrated). The baseline serum levels were 0.1 to 1 ng/mL and reached a peak of more than 100 ng/mL within 1 hour of intraperitoneal injection. Conclusions. This technique permits real-time measurement of intraprostatic pharmacokinetics of calcitriol. The ratio of the intraprostatic area under the concentration versus time curve to the serum area under the concentration versus time curve of calcitriol was less than 1:100. Hence, within 24 hours of calcitriol administration, only a fraction (less than 1%) of the serum level is detectable in prostatic tissue. A bimodal peak in intraprostatic calcitriol levels is observed. This technique could be used to determine the tissue levels of calcitriol more accurately and to conduct additional studies to better elucidate the effects of castration on the intraprostatic pharmacokinetics of calcitriol.