This field trial was performed to investigate the effects of chromium (Cr(+3)) on body weight, some serum parameters related to carbohydrate, lipid, protein and mineral metabolism, reproductive hormones and ovarian follicular activity of horses with poor body condition score. Twenty-four, quarter horses, aged from 4-13 years were evenly assigned into three groups. The animals received, orally, 0, 200 or 400 mu g Cr daily for 45 days in the form of chromium picolinate (CrPic). All of the animals consumed the identical diets consisting of concentrate and hay throughout the study. Initial and final body weights of the animals were recorded. The number and size of the follicle developed in the ovaries were measured by an ultrasonograph and plasma estradiol and progesterone levels were determined at weekly intervals. Other blood chemistry variables were determined at the end of the study. Chromium had no effect on body weight, serum glucose, total protein, globulin, High Density Lipoprotein (HDL), Low Density Lipoprotein (LDL) and mineral levels. Chromium increased serum albumin in both groups, but the increase in 200 mu g Cr supplemented group was significant (p<0.01). Both level of chromium slightly reduced total cholesterol concentration. A decrease was determined in triglycerides by 400 mu g Cr (p<0.05). Serum chromium level was increased by 400 mu g Cr (p<0.01). Follicle numbers and follicle size were slightly increased by 200 and 400 mu g Cr respectively. In the treatment groups, a linear increase was determined in the estrus rate after the 4(th) week. In conclusion, changes in some biochemical parameters by 200 and 400 mu g Cr supplementations and slight increases in follicular activity suggest that chromium may be of importance in field application. However, further detailed studies with various levels of chromium for longer period may be valuable to determine the biological functions of chromium in reproduction of horses.