NEW FINDINGS: What is the central question of this study? Is the elevated tonic renal nerve activity induced by chronic angiotensin administration mediated by recruitment or increased firing frequency and does this occur via stress, chemoreflex or baroreflex pathways? What is the main finding and its importance? Long-term angiotensin treatment in rabbits elevates renal sympathetic nerve activity by recruitment of previously silent fibres. This was similar to the effect of chemoreflex stimulation, but not to stress or baroreceptor activation, suggesting that presympathetic pathways activated by angiotensin may be common to those activated by chemoreceptors. Modulation of sympathetic nerve activity involves control by the CNS of the amplitude of neural discharges, reflecting recruitment of neurons and their firing frequency. We tested whether elevated tonic renal sympathetic nerve activity (RSNA) induced by chronic angiotensin administration is mediated by recruitment or increased firing frequency and whether this is characteristic of the pattern observed with activation of stress, chemoreflex or baroreflex pathways. Conscious rabbits treated with angiotensin II for 12 weeks to increase blood pressure by 10-30 were subjected to stress (air jet), hypoxia (10 O2 + 3 CO2 ) and drug-induced changes in blood pressure to produce baroreflexes. Total RSNA and RSNA burst amplitude were scaled to 100 normalized units (n.u.) by the maximal response to smoke. After 12 weeks of treatment, blood pressure was 17 higher than baseline 68 +/- 1 mmHg (P = 0.02). Compared with sham treatment, total RSNA and burst amplitude were +82 (P <0.001) and 39 (P = 0.04) greater, but burst frequency was similar. Total RSNA increased during hypoxia (+38 from 4.9 +/- 0.7 n.u.), owing to greater amplitude, but not frequency. Air-jet stress increased total RSNA (+44 from 4.3 +/- 0.5 n.u.) and burst frequency (+21 from 5.4 +/- 0.7 bursts s(-1) ), but not amplitude. Angiotensin enhanced total RSNA responses to both air jet (+33 ) and hypoxia (+58 ), but only increased the amplitude response to air jet. The RSNA baroreflexes reset to the higher blood pressure, but amplitude or frequency was not differentially altered. Chronic angiotensin treatment elevated RSNA by recruitment of neurons, which is similar to chemoreflex stimulation, but not to stress or baroreceptor activation, suggesting that presympathetic pathways activated by angiotensin may be common to those activated by chemoreceptors.