This study sought to determine if nucleotide induced relaxation of the guinea-pig tenia coli could be antagonized by arylazido- aminopropionyl-ATP (AA-ATP) which is mediated by P2 purinergic receptors, and to determine how the stimulation response of the nonadrenergic, noncholinergic nerves, by blockade of P2 receptors, affects the relaxation of the tissue. Tenia coli were removed from male guinea-pigs and placed in a 2 milliliter organ chamber through which modified Krebs-Henseleit solution was suffused. Transducers were used for measurement of isometric tension responses. The effects of treatment with 0.1 millimolar AA-ATP on the responses of the tissues to exogenously added agonists (ATP, ADP, and AMP) showed that concentration response curves for ATP, ADP, AMP, and adenosine were shifted in a parallel manner to the right of control, with maximal responses remaining unaffected. Responses to ATP were the most readily antagonized. AA-ATP did not antagonize responses to isoproterenol. The results indicate that the antagonism by AA-ATP occurred at the P2 receptors whether they mediate contraction or relaxation. When untreated tissues were exposed to exogenous ATP and to field stimulation (rectangular wave pulses, 40 volts, 5 milliseconds), AA-ATP had no effect on responses to the electrical stimulation but markedly reduced the responses to ATP. The authors conclude that the results suggest that the transmitter responsible for mediating responses resulting from stimulation of the nonadrenergic, noncholinergic motor nerves in the tenia coli was not ATP or a related adenine nucleotide or adenosine.