Benzaldehyde semicarbazone (BS) inhibited zymosan writhing response, carrageenan paw edema and both phases of formaldehyde nociceptive response. 2-hydroxybenzaldehyde semicarbazone (2-OHBS) and semicarbazide inhibited carrageenan paw edema and the second phase of formaldehyde nociceptive response. 2-OHBS inhibited zymosan writhing response. 3- and 4-OHBS did not show such activities. 2-OHBS showed the lowest LUMO energy, the highest contribution of the iminic carbon to LUMO energy, the highest positive charge on the iminic carbon, the highest negative charge on the iminic nitrogen and the highest susceptibility to hydrolysis. Hence semicarbazide may play important roles in 2-OHBS's activities. Inhibition of the first phase of formaldehyde response by BS could be attributed to its higher hydrophobicity and lower susceptibility to hydrolysis in comparison to 2-OHBS.