The effect of adenosine on an internal calcium store which can be released by carbachol to produce a transient contraction in Ca2+-free solution was investigated in the taenia depolarized by high K+. The carbachol contraction in Ca2+-free solution was increased by the preceding application of adenosine (3 X 10(-5) to 10(-3) M), an effect which was mimicked by ATP, but not by the slowly degradable analog alpha, beta-methylene ATP. The P1-purinoceptor antagonist 8-phenyltheophylline inhibited the increase caused by adenosine without modifying the carbachol contraction in controls. It is concluded that stimulation of an extracellular P1-purinoceptor increases an internal store of calcium, which might contribute to the relaxation induced by adenosine in the taenia.