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1. Measurements of contractions, conduction velocity and intracellular potential were made on isolated rabbit atria under four sets of conditions: high bicarbonate/high CO₂ (HH), low bicarbonate/low CO₂ (LL), high bicarbonate/low CO₂ (HL) and low bicarbonate/high CO₂ (LH). The ratio high/low was the same for the bicarbonate and CO₂ concentrations, so that HH had the same pH as LL.

2. Acid solutions caused a fall of a few mV in the resting potential, but not in the overshoot. They reduced conduction velocity and rate of rise of the action potential. They depressed contractions, but prolonged the tail of the action potential.

3. Alkaline solutions caused the converse changes, but, with the exception of the effect on the duration of the action potential, the relation with pH was markedly alinear, in that a rise in pH had much less effect than an equivalent fall.

4. Statistical tests were devised to decide whether the observed changes were associated primarily with pH, P_CO2 or bicarbonate. By far the strongest association was with external pH. Changes in P_CO2, per se, had no significant effect.