This was an investigation of the relationship between heart rate variability (HRV) and driver performance, and a preliminary test of an Experimental Alertness Indicator (EAI)- a device for measuring HRV. Three drivers drove on a round-the-clock basis for 5 days over a 364 mile (586 km) circuit on a California highway. HRV and driver error frequency were recorded and analyzed to determine effects of driving time, rest breaks, traffic event frequency, and other variables. The results showed that HRV increased markedly with driving time, HRV recovered after rest, HRV might have reflected features of the highway's geometric configuration, HRV dropped substantially after the occurrence of events which realerted the drivers, and HRV was little influenced by traffic event frequency per se. It was concluded that HRV is related to driver alertness/fatigue and that the EAI has promise of being useful for estimating the level of driver alertness.