In a double-blind, controlled, randomized, four-arm, bicentric clinical study, the effect of four oral contraceptives (OCs) on thyroid hormone parameters, cortisol, aldosterone, endothelin-1 and angiotensin II was investigated. Four groups composed of 25 volunteers each (ages between 18 and 35 years) were treated for six cycles with monophasic combinations containing 21 tablets with either 30 microg ethinylestradiol (EE) + 2 mg dienogest (DNG) (30EE/DNG), 20 microg EE + 2 mg DNG (20EE/DNG), 10 microg EE + 2 mg estradiol valerate (EV) + 2 mg DNG (EE/EV/DNG) or 20 microg EE + 100 microg levonorgestrel (LNG) (EE/LNG). The study was completed by 91 subjects. Blood samples were taken by venipuncture after at least 12 h fasting on Day 21-26 of the control cycle and on Day 18-21 of the first, third and sixth treatment cycle. There was a significant increase in triiodothyronine (T3) and thyroxine (T4) by 20-40% in all treatment cycles, while thyroid-stimulating hormone was significantly increased only with EE/EV/DNG. Treatment with the DNG-containing OCs caused no change in free T4 (FT4) and a transitory reduction in free T3 (FT3) levels during the first cycle. During intake of EE/LNG, FT4 rose slightly, while FT3 was not altered. The pronounced rise in the serum concentrations of cortisol appeared to be related to the EE dose. During the first three cycles of treatment, no effect on angiotensin II levels was observed, while in the sixth cycle a significant decrease was measured in all treatment groups. The four OCs did not influence the serum concentrations of endothelin-1 and no consistent effects were found concerning those of aldosterone. The results suggest that the three DNG-containing and the LNG-containing low-dose OCs may increase T3, T4 and cortisol due to an elevated binding to serum globulins, while the free proportion of the hormones is not or only slightly changed. Therefore, these OCs have only minor effects on thyroid function, adrenal and blood pressure serum parameters.