In this research we evaluated the calcium channel antagonist activity of various diester analogues of nifedipine on rat ileal smooth muscle. In these analogues, the orthophenyl group at position 4 was replaced by 1 methyl 2-meythylsulfonyl or methylthio 5- imidazolyl. Wistar rats (180-250g) were killed by a blow to the head. The intestine was removed above the ileucecal junction and longitudinal smooth muscle segments of 2cm length were maintained at 37°C in a 10 ml jacket organ bath containing oxygenated intestinal krebs solution. The contractions were recorded with a force displacement transducer connected to a physiographic. The contraction was elicited with 80 mmol KCL. Test compounds were cumulatively added to produce 50% relaxation of contracted ileal smooth muscle (IC50) that was determined from the concentration response trace recorded by physiograph. The IC50 ofnifedipine was (1.26±0.37) x 10-9 and of compounds 1, 2, 3, 4, 5 and 6 was (2.57±0.28) x 10-5, (1.03±0.12) X 10-5, (2.55±0.50) X 10-6 (1.32±0.18) X 10-9, (3.16±0.89) X 10-6 and (1.04±0.29) x 10-7 mole respectively. The results indicate that replacement of 2- nitrophenyl at C4 position of nifedipine with methylthio or methyl solfunyl imidazolyl reduces the activity. The comparison of the activities of symetrical esters (compounds No 3 & 6) indicates that increasing the length of methylen chain in C3 and C5 esters substituent. decreases the activity. Comparison of the activities of asymmetrical esters (compounds No 4 & 5) indicates that, when at C3 there is a small substituent, increasing the length of methylen chain increases activity. Comparison of the activites of symetrical esters (compounds 2 and 3) with asymetrical esters (compounds 1, 4, 5 and 6) indicates that asymetrical esters are not al-rvays more potent than symmetrical esters. Compound 4 was the most potent new compound in this study. Finally we can conclude that nifedipine was significantly more potent than all of these compounds.