Hamelia patens JAQC. (Rubiaceae) is a medicinal bush widely distributed in tropical areas of the American continent. It is used in Mexican Traditional Medicine for the treatment of menstrual disorders, therefore suggesting that its chemical constituents may have some effect on myometrium contractility. Physiological effects might differ due to quantitative variations in the content of alkaloids arising from its wide geographical distribution. To test this hypothesis, the content of oxindole alkaloids in methanol extracts of five different samples collected in Mexico was quantified by GC-MS. Each extract was assayed on contractility of estrogen-primed rat myometrium. Variations in the content of alkaloids were observed among the different samples. All samples relaxed in a concentration-dependent manner the high KCl-induced contraction in rat myometrium. Those which lack rumberine and/or maruquine displayed a higher relaxant effect than samples containing them, suggesting that these alkaloids might counteract the effects of isopteropodine. However, in contrast with verapamil, Hamelia patens metanol extracts are poor relaxants.