Previously, a mixture of conjugated linoleic acid (CLA) isoforms reduced parathyroid hormone (PTH) in male rats over 8 weeks. The objective herein was to determine which isoform caused the reduction in PTH; whether the effect was sex specific; and whether CLA-induced reductions in PTH were sustained. Male and female weanling rats (n=48) were randomized to a control diet or one made with 0.5% of the diet as cis-9, trans-11 (c9,t11) CLA, 0.5% of the diet as trans-10, cis-12 (t10,c12) CLA or these CLA in a mixture. Measurements made after 4, 8 and 16 weeks were body weight, bioactive PTH, ionized Ca, whole-body and regional bone mineral density (BMD) using dual-energy X-ray absorptiometry. With the use of a factorial design, a sexxc9,t11 CLA interaction was observed that reduced PTH (139.5+/-63.9 vs. 95.8+/-42.4 pg/ml, P=.02) in male rats only. No other effects of c9,t11 CLA were observed. Regarding t10,c12 CLA, no interaction effects were observed, but a main effect was observed to reduce lumbar spine BMD (0.265+/-0.044 vs. 0.255+/-0.044 g/cm(2), P<.01) along with reduced retention of Ca and P at Week 4. No other dietary effects were observed. In summary, the c9,t11 CLA isoform is responsible for reduced PTH and this effect is sex specific; this was true whether fed as a pure isomer or mixed with an equal amount of t10,c12 CLA. Whether such reductions in PTH might be observed in females lacking sex hormones such as ovariectomized rats and also in humans is required to expand health implications of dietary CLA.