1. Recordings were made over long periods of the ECG and pressure changes in the orobranchial (OB) cavity of unanaesthetized dogfish, kept in a closed circulation of aerated and temperature controlled sea water.

2. Variations in cardiac frequency (11-64/min) and in ventilatory frequency (22-80/min) were observed between individuals of similar body size. Similar variations were observed in non-experimental fish kept in holding tanks.

3. Analysis of tape-recordings using a Biomac computer was made for interval histograms and showed a greater variability of the ECG interval than of the OB pressure, but not at all times. The relationship between the two rhythms was further investigated using event correlograms showing the position of the ECG in an averaged ventilation cycle, which was divided into ten equal phases.

4. In many fish the heart tended to beat in a particular phase, at least for short periods, but in some cases this was never observed. Given individuals also varied is the preferred phase. Complete synchrony was rarely observed, most fish showing evidence of varying degress of coupling between the rhythms.

5. Analysis of this incomplete synchrony was carried out using polar co-ordinates which showed that in 60% of the recordings analysed there was some significant (1% level) coupling between the rhythms. The phase angle varied considerably and was most commonly between 0° and 180°, i.e. during the first half of the cycle following the maximum positive OB pressure.

6. It is concluded that further understanding of the relationship between these rhythms requires more detailed knowledge of flow patterns of blood and water across the secondary lamellae.

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