Fig. 2. Spontaneous heart rate f_H of 21°C normoxia- and anoxia-acclimated right-atrial preparations during exposure to various saline solutions devised to simulate or reverse the changes occurring in turtle blood plasma during 6 h of anoxic submergence, in the presence of 1 or 10 nmol l^–1 tonic adrenaline (indicated in parentheses). Exposure order for normoxia-acclimated preparations was as given in the figure, reading from left-to-right, whereas exposure order for anoxia-acclimated preparations was as reading from right-to-left. Significant differences (P<0.05) in f_H among saline solutions within an experimental group are indicated by different letters. Significant differences (P<0.05) in f_H from a comparable Control protocol f_H. Significant differences (P<0.05) in f_H between 1 nmol l^–1 and 10 nmol l^–1 tonic adrenaline groups within an acclimation condition (i.e. normoxia- or anoxia-acclimated). ^*Significant difference (P<0.05) in f_H between normoxia- and anoxia-acclimated preparations (of the same level of tonic adrenergic stimulation) under simulated in vivo normoxic or simulated in vivo anoxic conditions. Values are means ± s.e.m.; N=6 in the 1 nmol l^–1 tonic adrenaline normoxia- and anoxia-acclimated groups, 5 in the 10 nmol l^–1 tonic adrenaline normoxia-acclimated group and 6 in the 10 nmol l^–1 tonic adrenaline anoxia-acclimated group.