Globally, primary dysmenorrhoea is one of the most frequent gynaecological disorders in young women. It is associated with increased uterine tone, and exaggerated contractility of uterine smooth muscles. In many rural African communities, a number of medicinal plants, including Psidium guajava Linn. (family: Myrtaceae), are used traditionally for the management, control and/or treatment of primary dysmenorrhoea. The present study was, therefore, undertaken to examine the spasmolytic effect of Psidium guajava leaf aqueous extract (PGE) on isolated, spontaneously-contracting and oestrogen-dominated, quiescent uterine horns of healthy, young adult, female Wistar rats. Graded, escalated concentrations of PGE (0.5-4.0 mg/ml) produced concentration-dependent and significant inhibitions of the amplitude of spontaneous phasic contractions of the isolated rat uterine horn preparations. In a concentration-related manner, PGE also significantly inhibited or abolished contractions produced by acetylcholine (ACh, 0.5-8.0 microg/ml), oxytocin (0.5-4.0 microU), bradykinin (2.5-10 ng/ml), carbachol (CCh, 0.5-8.0 microg/ml) or potassium chloride (K+, 10-80 mM) in quiescent uterine horn preparations isolated from the oestrogen-dominated rats. The spasmolytic effect of PGE observed in the present study lends pharmacological support to the traditional use of ;guava' leaves in the management, control and/or treatment of primary dysmenorrhoea in some rural African communities.