The fruit of Momordica charantia (family: Cucurbitacea) is used widely as a hypoglycaemic agent to treat diabetes mellitus (DM). The mechanism of the hypoglycaemic action of M. charantia in vitro is not fully understood. This study investigated the effect of M. charantia juice on either 3H-2-deoxyglucose or N-methyl-amino-a-isobutyric acid (14C-Me-AIB) uptake in L6 rat muscle cells cultured to the myotube stage. The fresh juice was centrifuged at 5000 rpm and the supernatant lyophilised. L6 myotubes were incubated with either insulin (100 nM), different concentrations (1-10 microg ml(-1)) of the juice or its chloroform extract or wortmannin (100 nM) over a period of 1- 6 h. The results were expressed as pmol min(-1) (mg cell protein)(-1), n = 6-8 for each value. Basal 3H-deoxyglucose and 14C-Me-AIB uptakes by L6 myotubes after 1 h of incubation were (means +/- S.E.M.) 32.14 +/- 1.34 and 13.48 +/- 1.86 pmol min(-1) (mg cell protein)(-1), respectively. Incubation of L6 myotubes with 100 nM insulin for 1 h resulted in significant (ANOVA, p < 0.05) increases in 3H-deoxyglucose and 14C-Me-AIB uptakes. Typically, 3H-deoxyglucose and 14C-Me-AIB uptakes in the presence of insulin were 58.57 +/- 4.49 and 29.52 +/- 3.41 pmol min(-1) (mg cell protein(-1)), respectively. Incubation of L6 myotubes with three different concentrations (1, 5 and 10 microg ml(-1)) of either the lyophilised juice or its chloroform extract resulted in time-dependent increases in 3H-deoxy-D-glucose and 14C-Me-AIB uptakes, with maximal uptakes occurring at a concentration of 5 microg ml(-1). Incubation of either insulin or the juice in the presence of wortmannin (a phosphatidylinositol 3-kinase inhibitor) resulted in a marked inhibition of 3H-deoxyglucose by L6 myotubes compared to the uptake obtained with either insulin or the juice alone. The results indicate that M. charantia fruit juice acts like insulin to exert its hypoglycaemic effect and moreover, it can stimulate amino acid uptake into skeletal muscle cells just like insulin.