Xanthorrhizol, xanthorrhizol glycoside, and trachylobanoic acid, compounds isolated from medicinal plants that are grouped in the complex known as Cachani have been shown to inhibit the tonic contraction of rat uterus induced by: (a) depolarizing K+ solution (60 mM), (b) CaCl2 (1 mM), and (c) BAY K 8644 (0.3 microM) in a concentration-dependent manner (1-30 micrograms/mL). The inhibitory potency was displayed as follows: xanthorrhizol > xanthorrhizol glycoside > trachylobanoic acid. These results suggest that the assayed compounds might block voltage operated calcium influx in myometrial cells as they displayed a calcium-antagonistic activity. This effect is not due to peripheral receptor activation (beta 2-adrenergic, H2-histaminergic) as neither propranolol nor cimetidine modified the inhibitory effect of the compounds assayed. This is the first report showing that plants belonging to the Cachani complex may contain uterine smooth muscle bioactive substances.