Abstract: This study was performed for investigating of the effects of tamsulosine and doxazosine in vitro on trigone smooth muscle of rat bladder. Fifty rats which were 250-300 gr were used for this study. One strip shape trigone preparat was prepared for each of the isolated bladder. At first, 1 gram of strecth was applied to tissues and after waiting for training to media. Next, determination of the level of electrical stimulation which created submaximal contraction after training time and this stimulation was performed with adding different concentrations of phenylephrine. At the end of this process, effective dosage was found, for trigone was determined by applying different concentrations of phenylephrine (10-8 M, 10-7 M, 10-6 M, 10-5 M) respectively. Firstly 10-9 M dosage of doxazosine was added as a adrenergic receptor antagonists which tamsulosine (10-8M, 10-7M, 10-6M, 10-5M) and doxazosine (10-9M, 10-8M, 10-7M, 10-6M) and waited for 20 minute. Then, effective dosage of phenylephrine (10-5 M) was added to the solution and waited for 7 minute, again. After this process, electrical stimulation was applied for contraction of the tissue. After stimulation, tissue was washing once for every two minute twice and by resting, tissue was waited until it got its starting strecting value. Same processes were performed for tamsulosine and doxazosine other dosages. As a result, when we compared the amplitutes of the responses of all concentrations of doxazosine at trigone smooth muscle with amplitute of a response of effective concentration of phenylephrine. Alone, a decrease in the amplitites were found in the ration of 10 % (10-9M), 14 % (10-8M) 21 % (10-7M), 28 % (10-6M) respectively related to increase of concentration. Similarly, when we compared the amplitutes of the responses of all concentrations of tamsulosine with amplitute of a response of effective concentration of phenylephrine. Alone, a decrease in amplititues were found in the ration of 32 % (10-8M ), 37 % (10-7M) 41 % (10-6M), 46 % (10-5M) respectively related to increase of concentration. Prevention of contractions which was done with tamsulosine hydrochloride more effectively than doxazosine mesylate was determined. With these results, we showed that doxazosine and tamsulosine inhibited noradrenaline based contractions at the rat trigone smooth muscle and this results can be used both for in vitro and in vivo for future studies.