In critical illness, several drugs and various stressful conditions modify the functions of neurotransmitters which consequently affect the secretion of pituitary hormones. Although the role of neurotransmitters in the regulation of endocrine system is well known, cholinergic actions have been less investigated. In animals, cholinesterase inhibitors were shown to modify the pituitary-thyroid and pituitary-adrenal axes, and to affect prolactin levels. The aim of the present study was to determine the effect of the organophosphate compounds on endocrine system, particularly pituitary hormones. This prospective study was performed in Medical Intensive Care Unit of Erciyes University Medical School Hospital. Twenty-two consecutive patients (ten males and 12 females aged 28+/-8 years) with organophosphate poisoning were included in the study. ACTH (P<0.002), cortisol (P<0.0005) and PRL (P<0.005) levels were significantly higher during poisoning than after resolution of poisoning. FSH levels were significantly lower during poisoning (P<0. 05). Sick euthyroid syndrome was determined in seven patients (31. 8%). Two of them had low fT3 (with normal fT4 and TSH), two had low fT4 (with normal fT3 and TSH) and three had low TSH (with normal fT3 and fT4) levels. Serum levels of these hormones returned to normal values after resolution of poisoning. The present study demonstrated that organophosphate compounds affected PRL, ACTH and cortisol levels, but did not change LH levels. Organophosphate compounds may result in sick euthyroid syndrome. These conditions may be related to the effects of acetylcholine and direct effect of organophosphate compounds.