Microelectrode recordings of muscle nerve sympathetic activity (MSA) in man have shown a reflex relationship between heart beat and corresponding sympathetic burst, the latency of which is stable at rest and independent of heart rate. In peroneal nerve recordings in 35 healthy subjects this latency was reduced during the Valsalva manoeuvre by 120 ms (mean; range 40-245 ms; P less than 0.001) from a mean value at rest of 1300 ms. Slow deep breathing and simulated diving shortened the latency by 60 (P less than 0.001) and 80 ms (P less than 0.05), respectively. When intrinsic heart rate was induced by i.v. administration of atropine and propranolol, the latency was increased by 70 ms (P less than 0.001). A number of other manoeuvres affecting the outflow of MSA did not change the latency. It is suggested that the findings indicate the existence of more than one central pathway involved in the baroreflex regulation of MSA. Alternatively, altered central processing time may follow influence from other receptors in different manoeuvres.