The aim of the present study was to investigate the incidence and causes of ovarian inactvity and evaluate the efficacy of single administration of GnRH for treatment of inactive ovaries in dairy cows. Two experiments were carried out to fulfill the aims of the study. In experiment 1, 122 cows which had not shown estrous signs ³ 45 after calving were examined twice by transrectal ultrasonography at a 7-10 day interval to diagnose the causes of anestrous and assess the incidence of ovarian inactivity. In Experiment 2, blood samples were collected from 30 cows with ovarian inactivity and 13 cyclic cows to compare the blood concentration of insulin, glucose, calcium and phosphorus between the two types of cows. Cows (n=30) suffering from ovarian inactivity were treated with either 500 mg gonadorelin acetate intramuscular (G1; n=20) or saline intramuscular (G2; n=10). The ovaries of cows in G1 and G2 were examined by transrectal palpation twice before and twice after the treatment at a week interval. After transrectal palpation, blood samples were collected from cows for assessment of progesterone, insulin and glucose. Also the cows were observed for estrous signs and those cows showed estrus were inseminated. In Experiment 1, silent or unobserved estrus (47.5%), inactive ovaries (41%), cystic ovaries (9%) and pyometra (2.5%) were the reasons of anestrous in the studied dairy cows. In Experiment 2, there were significant differences in the blood concentrations of insulin (P< 0.0005) and glucose (P< 0.01) between cyclic and non-cyclic cows. On the other hand, there were non-significant differences in the blood concentrations of calcium and phosphorus between the cyclic and non-cyclic cows. Estrus induction rates within 14 days after treatment were significantly (P< 0.05) higher in G1 (60%) than in G2 (10%). Seven of 12 cows (58.3%) responded to administration of GnRH in G1 and the responded cow (1/1) in G2 were pregnant at Day 60 after AI. In conclusion, ovarian inactivity is the main reason of postpartum anestrous in dairy cows. Lower level of insulin and glucose might be among the reasons of ovarian inactivity. Single administration of GnRH is reasonably effective for treatment of ovarian inactivity in dairy cows.