Fig. 1. The effect on the blood acid–base status of African lungfish Protopterus annectens of 1 h of (A–D) acid (3 mmol kg^–1 NH₄Cl) or (E–H) base (3 mmol kg^–1 NaHCO₃) infusion. Acid–base status prior to and following the infusion period (marked by the grey bar) was assessed by determining the pH (A,E), HCO₃^– ion concentration ([HCO₃^–]) (B,F) and CO₂ tension (Pa_CO2) (C,G) of arterial blood. (D,H) pH–HCO₃^– diagrams are used to summarize the effect of acid or base infusion on acid–base status. Values are means ± s.e.m.; N=11 for acid-infused fish and N=6–8 for base-infused fish. An asterisk (^*) indicates a significant difference from the pre-infusion value (one-way repeated measures ANOVA; P values are indicated on the figure). For pH–HCO₃^– diagrams, the P_CO2 for a given combination of pH and [HCO ^–₃] was calculated using the Henderson–Hasselbalch equation and the appropriate values for pK' and Pa_CO2 (Boutilier et al., 1984). The buffer line was constructed using data for Protopterus aethiopicus (Lenfant and Johansen, 1968). Pre, period prior to acid or base infusion.