This work provides substantial evidence for the advocated diuretic effect of parsley in folk medicine and determines the mechanism of action of the herb. Rats offered an aqueous parsley seed extract to drink, eliminated a significantly larger volume of urine per 24 h as compared to when they were drinking water. These findings were supported by the results of other experiments using an in situ kidney perfusion technique which demonstrated also a significant increase in urine flow rate with parsley seed extract. This effect was still apparent in presence of amiloride, furosemide and in the absence of sodium, but not in the absence of potassium, suggesting that the diuretic effect of the herb is mediated through an increase in K+ retention in the lumen. Parsley extract, was shown on the other hand, to reduce the activity of the Na+-K+ ATPase in both cortex and medulla homogenates. Such an inhibition would decrease apical cellular Na+ reabsorption, lower K+ secretion, increase K+ concentration in the intercellular space and consequently would inhibit passive K+ influx across the tight junctions. The mechanism of action of parsley seems to be mediated through an inhibition of the Na+-K+ pump that would lead to a reduction in Na+ and K+ reabsorption leading thus to an osmotic water flow into the lumen, and diuresis.