The acaricide chlordimeform (CDF) has been reported to have effects on the central nervous system that appear to involve an interaction with alpha-adrenergic receptor-mediated mechanisms of neurotransmission. The present study examined the influence of CDF on pituitary-adrenocortical hormonal secretion, since there is a well-established alpha-adrenoceptor component to this activity. Effects on prolactin (PRL) were also evaluated, as its release is often concurrent with adrenocortical hormonal secretion and may involve an adrenergic contribution. CDF-injected (20 or 50mg/kg) male Long-Evans rats were killed after 1,4,8 or 24 hrs. Both non-injected and saline-injected controls were included. Dosing was structured so that trunk blood could be collected during the morning nadir of circulating corticosterone (CORT). Plasma adrenocorticotropic hormone (ACTH), CORT, and PRL all increased sharply by 1 hr following 50 mg/kg CDF. CORT increased in a dose-dependent fashion and declined over the ensuing 8 hrs. Separate groups of animals were also pre-treated with the alpha-adrenoceptor antagonist phenoxybenzamine (PBZ, 20mg/kg) or the alpha-receptor agonist clonidine (CLON, 0.6mg/kg) 40 min before and killed 1 hr after CDF (25mg/kg) injection.