Preeclampsia (PE) is a pregnancy-related disorder that significantly increases the risk of maternal and fetal morbidity and mortality. Melatonin, a potent antioxidant, has been suggested to mitigate oxidative stress and associated damage in various pathological conditions. Placental growth factor (PlGF) plays a vital role in placental development by promoting angiogenesis. This study aimed to investigate whether the levels of melatonin, cytokines, and PlGF were higher in the venous blood of women with preeclampsia during the third trimester of pregnancy compared to those with uncomplicated pregnancies. The study involved 32 women with preeclampsia and 33 healthy pregnant women as a control group. The concentrations of melatonin and PlGF were significantly lower in women with preeclampsia compared to healthy pregnant women. Specifically, the mean level of melatonin in the preeclampsia group was 30.98 pg/ml and 55.20 pg/ml in the control group (p=0.029). Similarly, the mean level of PlGF in the preeclampsia group was 40.03 pg/ml and 213.31 pg/ml in the control group (p<0.0001). This suggests that alterations in the placental production of melatonin and PlGF may contribute to the development of preeclampsia. In contrast, we observed higher levels of the pro-inflammatory cytokine interleukin-6 (IL-6) and the anti-inflammatory cytokine interleukin-10 (IL-10) in the preeclampsia group than in the control group. The mean concentration of IL-6 in the PE group was 270.79 pg/ml, whereas the control group had 224.30 pg/ml (p=0.022). Similarly, the mean concentration of IL-10 in the PE group was 41.90 pg/ml and 30.73 pg/ml in the control group (p=0.018). In women with uncomplicated pregnancies, the interaction between pro-inflammatory interleukine-6 and melatonin can be described by equality of statistical regression.