A double‐barrelled, pH‐sensitive micro‐electrode suitable for use in mammalian smooth muscle has been developed. It was shown to be unaffected by alteration of Na, K, Ca, Mg, Cl or CO2, and yielded the same results in mouse soleus muscle as had been obtained previously with the recessed‐tip pH‐sensitive glass electrode ( Aickin & Thomas, 1977 a). Intracellular pH (pHi) of the surface cells of guinea‐pig vas deferens was 7.06 +/‐ 0.09 (n = 52, S.D. of an observation) in Krebs solution equilibrated with 3% CO2 at pH 7.35. The membrane potential (Em) was ‐65.5 +/‐ 6.7 mV. Thus pHi is about 0.8 units more alkaline than that predicted if H+ ions were passively distributed across the cell membrane. Alteration of extracellular pH (pHo) at constant CO2 caused a smaller change in pHi, by about 40% of that in pHo. The change was complete in 6‐12 min and was of a similar magnitude whether the alteration was made in the continual presence or absence of CO2. Alteration of the CO2 level at constant pHo caused a rapid change in pHi followed by a slower, complete recovery. Thus the same stabilized pHi was recorded in different CO2‐containing solutions. When CO2 was removed, the expected intracellular alkalinization was reduced or even obscured by a considerable acidification. pHi then stabilized at a mean value of 6.81 +/‐ 0.11 (n = 18) with an Em of ‐60.8 +/‐ 8.2 mV. The acidification expected on readmission of CO2 was minimized or obscured by a rapid recovery of pHi to the value previously recorded in CO2‐containing solutions. A simultaneous increase in CO2 and decrease in pHo caused a rapid fall in pHi which increased in magnitude with decreasing pHo. This fall was followed by an incomplete recovery when pHo was above about 6.8 (but below 7.35), by no further change in pHi when pHo was about 6.7 and by a slow, continued fall in pHi when pHo was below 6.7. The intrinsic buffering power was calculated from the pHi changes observed on alteration of CO2. The values obtained increased in parallel with the extent to which pHi recovered following the imposed change, probably explained by an inseparable contribution to the minimization of the change by transport processes.(ABSTRACT TRUNCATED AT 400 WORDS)