Twenty-six epoxide and corresponding pyrazole derivatives, of the structurally related chalcones and combretastatin A-4 (CA-4), were synthesized and tested for in vitro cytotoxicity. These molecules were synthesized by epoxidation of the relevant chalcones, followed by reaction with hydrazine. The structures of epoxides 3 and 7, and pyrazole 17, were confirmed by X-ray diffraction studies. The relatively coplanar conformation of a 3',3'',4',4'',5',5''-hexamethoxypyrazole 17 was in good agreement with the shape for 3',3'',4',4'',5'-pentamethoxypyrazole 16, which was determined from molecular mechanics optimization. In vitro cytotoxicity of each class of compounds was obtained using a 72 h continuous exposure MTT assay against two murine cancer cell lines; B16 melanoma and L1210 leukemia. The effect of substitution in the A-ring is addressed: three methoxy groups versus two, generally increased cytotoxicity across both cell lines. In the majority of cases, the pyrazoles are generally more active than the epoxides, with the most active, 5-(3''-amino-4''-methoxyphenyl)-3-(3',4',5'-trimethoxyphenyl)pyrazole 21, possessing an IC(50) value of 5 and 2.4 microM (B16 and L1210, respectively). Due to their planar conformations, the pyrazoles are typically less active than the corresponding chalcones, which adopt angular conformations similar to CA-4. B-ring modifications confirmed that in general the amino compounds are more active than the corresponding nitro compounds. Varying the number and orientation of methoxy groups on the A-ring did not produce any significant differences in toxicity in the cell lines studied.