The purpose of this study is to establish a protocol of retention-enema experiments and evaluate the antihypertensive effect and the safety of Gwakhyangjeonggi-san retention enema. Normal and spontaneously hypertensive rats (SHRs) were divided into treatment and control groups, respectively. We applied the Gwakhyangjeonggi-san extract by decoction and 0.9% NaCl in each group, estimated the blood pressure and body weight, and performed HPLC analysis. ALT, AST, BUN, and creatinine were examined. The systolic blood pressure within each group in normal rats differed significantly in time effect, and so did the diastolic blood pressure in the treatment group of normal rats. The systolic, diastolic, and mean blood pressure showed significant differences in group effect in the treatment group of the SHRs. The time effect of the body weight in both groups of normal rats differed significantly, so did group × time and time effects in both groups of SHRs. AST, ALT, BUN, and creatinine showed no significant difference between groups. We concluded that the Gwakhyangjeonggi-san retention enema has a hypotensive effect in normal rats within the regular range of blood pressure, but an antihypertensive effect in SHRs. Also, the intervention is safe and does not affect the liver and kidney functions in normal rats.