The midcycle surge of luteinizing hormone (LH) from the pituitary provides the physiological trigger in the mammalian female for the process of ovulation. Accordingly, any agent that compromises the LH surge could function as a reproductive toxicant. The acaricide chlordimeform (CDF) has previously been found to decrease serum LH, probably by altering the hypothalamic noradrenergic transmitter control of LH secretion. Consequently, the present study focused on the effect of acute CDF administration on the appearance of the induced LH surge. Single intraperitoneal injections of CDF (0, 10, 25, 50 mg/kg) in OVX, estradiol - implanted female Long-Evans rats approximately 5 hrs prior to the expected surge caused a complete suppression at 25 and 50 mg/kg. Ten mg/kg had no effect on surge amplitude, but advanced the LH peak by 2 hrs. Since CDF has been found to elevate serum corticosterone (CORT), 10 mg CORT/rat were given at different times prior to the surge. Following 20 hrs of CORT exposure, only a partial lowering was seen; 5 hrs exposure were ineffective, indicating an indirect adrenal effect was not the principal route, but may accompany an action of CDF on the hypothalamic mechanisms regulating the surge.