A series of 17 analogs of 1-acetyl-4,5-dihydro(1H)pyrazoles (JP-1 to JP-17) bearing two aromatic rings at positions 3 and 5, either of which ought to be heterocyclic, were synthesized and evaluated for their anti-proliferative potential against breast cancer (MCF-7 and T-47D) and lung cancer (H-460 and A-549) cell lines for the first time. JP-1-7, -10, -11, -14, and -15 were observed to exhibit significant anti-proliferative activity against MCF-7 cells. Some notions about structure-activity relationships are reported. The investigated compounds were found to lower the intracellular reactive oxygen species in the H2DCFDA assay and also caused mitochondria-dependent cell death in the MCF-7 cell line, indicating a plausible mechanism of their anticancer effect. Analogs of 1-acetyl-4,5-dihydro(1H)pyrazoles (JP-1-17) were synthesized and evaluated for their anti-proliferative activity in four cancer cell lines and for their intracellular ROS scavenging properties. An attempt was made to determine the mitochondrial membrane potential of MCF-7 cells treated with JP-1 and -14, aiming to elucidate the mechanism by which proliferation was curbed. ? 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.