Phytochemical investigation of the roots of Aconitum pseudostapfianum resulted in the isolation of two new C19-diterpenoid alkaloids, designated as pseudostapines A and B (1 and 2). Their structures were determined by spectral methods such as 1D and 2D (H-1-H-1 COSY, HMQC, NOESY, and HMBC) NMR spectroscopy, in addition to high-resolution mass spectrometry. The isolated alkaloids were tested in vivo for antinociceptive potential. The results show that pseudostapines A and B possess more potent antinociceptive effect than the positive control drugs aspirin and acetaminophen.