1. 1. We have investigated the effect of various protein kinase A (PKA) inhibitors on the phasic and tonic components of the response to potassium chloride (KCI) in the guinea pig ureter. All experiments were performed in ureters pretreated with capsaicin (10 μM for 15 min) to prevent the release of sensory neuropeptides and in the presence of 1 μM Bay K 8644 to maximize calcium (Ca) entry via voltage-sensitive channels. The addition of 80 mM hypertonic KCI produced maximal shortening of the ureter with distinct phasic and tonic components, the latter further showing a transient and a sustained component. Nifedipine (30 μM for 120 min) totally abolished all the responses to KCI. 2. 2. The selective PKA inhibitor, H89 (10 μM), abolished the tonic response to KCI in about 30 min with minor inhibitory effect on the phasic contraction. This pattern was unchanged when extending the contact time to 120 min. When added 30 min before the next challenge, H89 (1–30 μM) concentration-dependently inhibited the responses to KCI with a preferential inhibitory effect on the tonic contraction. Another PKA inhibitor, H8, produced similar effects at tenfold higher concentrations (10–300 μM) than H89, consistent with the known potency ratio of these isoquinoline derivatives in inhibiting PKA. 3. 3. The potent and nonselective protein kinase inhibitor, staurosporine (10–100 nM) produced an even depression of the various phases of the response to KCI. The selective protein kinase G inhibitor, KT 5823 (10 μM for 60 min) produced only a slight reduction of the sustained tonic response to KCI. The selective protein kinase C inhibitor GF 109,203X (1–3 μM) and the cAMP analog, Rp-cAMPS (300 μM for 60 min) had no effect on the three components of the response to KCI. 4. 4. In the presence of Bay K 8644, electrical field stimulation (10 Hz for 1 sec, 60 V, pulse width 5 ms) produces direct myogenic phasic contractions (twitches) of the ureter which are suppressed by nifedipine (10–30 μM). H8 (up to 30 μM) and H89 (up to 300 μM) had minor effect on the amplitude of twitches, consistent with their poor inhibitory activity on the phasic responses to KCI. 5. 5. In sucrose gap, superfusion with 80 mM hypertonic KCI produced action potentials followed by a sustained depolarization of the membrane: the two electrical responses underlie the phasic and tonic components of contraction to KCI, respectively. H89 (10 μM for 30 min) did not affect the resting membrane potential nor the KCl-evoked action potentials and sustained depolarization. H89 had no effect on the phasic contraction to KCl but markedly depressed (about 65% inhibition) the tonic contraction. 6. 6. The present findings are consistent with the view that phosphorylation by PKA increases the availability of L-type Ca channels in the ureter smooth muscle. Blockade of PKA dissociates the electromechanical coupling between the sustained membrane depolarization produced by KCI and the corresponding sustained increase in tension. The L-type Ca channel responsible for generating action potentials and phasic contractions to KCI are less sensitive to PKA inhibitors than those responsible for the tonic contraction.