Effects of total coumarins, essential oil and water extracts of Cnidium monnieri on plasma prostaglandin (PGE2 and PGF2 alpha) and cyclic nucleotide levels in rats of Kidney-Yang insufficiency were studied. 55 rats were divided randomly into 5 groups, Group I was administered orally with saline (normal group), group II was injected with intraperitonally hydrocortison acetate to induce Kidney-Yang insufficiency (control group), group III, group IV and group V (experimental groups) were injected with hydrocortison acetate, the same as group II, and administered orally with the total coumarins, essential oil and water extracts of Fructus Cnidii respectively. The levels of plasma PGE2, PG2 alpha and plasma cAMP, cGMP were measured. In group II, in comparing with those of group I, the levels of plasma PGE2, and PGF2 alpha decreased significantly (P < 0.01), and the value of cAMP/cGMP also lowered obviously (P < 0.01) due to the significant reduction of cAMP and insignificant change of cGMP. In group III and group V, the above-mentioned indices changed significantly (P < 0.01 or 0.05) compared with those of group II, and after treatment it normalized basically in comparing with those of group I. In group IV, those indices didn't change regularly and apparently as group III and group V did, compared with group II, and could not normalize satisfactorily. It is suggested that the coumarins in the fruit of Cnidium monnieri are probably the effective ingredients to invigorate Kidney and strengthen Yang, while the efficacy of essential oil remained unconfirmed.