The title compounds platyphylline (C(18)H(29)NO(5)0.5H(2)O) and integerrimine (C18H25NO6) have been isolated from the serial parts of the Senecio species. The crystal and molecular structures of platyphylline have been determined. The compound crystallises in orthorhombic space group, P2(1)2(1)2(1), The pyrrolizidine nucleus exhibits exo-buckling conformation. The molecular geometry was stabilized by C-H...O and C-H...N hydrogen bonds. A search for the low energy conformations of integerrimine as a similar alkaloid was also carried out. Force field calculations were undertaken by the MM3 program and some particular conformations were computed with the semi-emprical molecular orbital methods by the AMI and PM3 programs.