Increased haemodynamic regularity has been associated with various critical pathologies, e.g. sepsis. Recurrence quantification analysis (RQA) is a geometrical tool for the detection of recurrent structures and temporal correlations in time series, i.e. regularity. We studied the impact of glyceryl trinitrate on haemodynamic RQA values. With approval of the District Governmental Animal Research Committee, six anaesthetized and mechanically ventilated foxhounds (FiO2 0.3, etCO2 35 mmHg) received glyceryl trinitrate (GTN) at 1.5 mg kg-1 h-1 to reduce their systolic arterial pressure (SAP) to 80% of baseline values. RQA was performed on SAP and RR interval (RRI) time series (1000 cardiac cycles). SAP was reduced with GTN from 106 (98-136) mmHg to 87 (75-108) mmHg and recovered after 30 minutes washout to 94 (85-112) mmHg. RRI decreased from 734 (607-901) ms to 542 (485-636) ms and recovered to 651 (539-738) ms. With the reduction of blood pressure, the percentage of recurrent points (REC) of RRI increased significantly from 0.1 (0.0-1.0) to 7.4 (0.7-17.0) and remained at 1.9 (0.4-6.6) even after washout. By contrast, REC of SAP was not influenced significantly. In conclusion, glyceryl trinitrate has a marked and persistent influence on haemodynamic regularity. In patients, the use of glyceryl trinitrate may be one factor crucially influencing haemodynamic regularity.