The crude ethanolic extract prepared from the stem bark of Solanum pseudo-quina produced excitatory effects dominated by convulsions in rats and mice. Solvent extraction followed by alumina column chromatography resulted in the isolation of a pharmacologically active material (AP) which was identified to be (25S)-isosolafloridine. The convulsions produced by AP were predominantly clonic and invariably preceded by generalized fine and coarse tremors. This convulsive behaviour did not entirely resemble the convulsions produced by strychnine, pentylenetetrazol, bicuculline, picrotoxin or 3-mercaptopropionic acid. The tremor and convulsions were only slightly affected by drugs interfering with cholinergic, catecholaminergic, serotoninergic or encephalinergic neurotransmission. Only diazepam and particularly gamma-vinyl-GABA blocked AP-induced effects. After section of the spinal cord at a mid-theoretic level, AP produced convulsions only in the anterior part of the body. After intracerebroventricular administration, AP produced only sedation. A depressive effect was also observed on the blood pressure of conscious rats before and after the convulsions. In subconvulsive doses AP enhanced spontaneous motor activity in mice.