BACKGROUND: The Threshold Limit Values (TLV) of the American Conference of Governmental and Industrial Hygienists indicate the levels of heat stress that all workers may be repeatedly exposed to without adverse health effects. In this study, we evaluated heart rate variability (HRV) during moderate-to-heavy work performed continuously or according to different TLV work-rest (WR) allocations in healthy physically active older workers. METHODS: Nine healthy older (58 +/- 5 years) males performed three different 120-minute conditions in accordance with TLV guidelines for moderate-to-heavy intensity work (360 W fixed rate of heat production) in different wet-bulb globe temperatures (WBGT): continuous cycling at 28 degrees C WBGT (CON), as well as intermitted work performed at WR of 3:1 in 29 degrees C WBGT (WR3:1), and at WR of 1:1 at 30 degrees C (WR1:1). Rectal temperature and HRV (3-lead electrocardiogram [ECG]) were assessed throughout. RESULTS: Coefficient of Variation, Poincare SD2, and Shannon Entropy were decreased during the CON compared with the WR3:1 when core temperature exceeded 38 degrees C and after 1 hour of continuous work (P < .05). Also, 4 of the 12 HRV indices studied were reduced at CON compared with WR1:1 after 2 hours of accumulated work time (P < .05). Participants worked longer before core temperature reached 38 degrees C during the WR1:1 and the WR3:1, compared with CON (P < .05). CONCLUSIONS: Incorporating breaks during moderate-to-heavy work in the heat for older adults can reduce autonomic stress and prolong the work performed at safe core temperature levels. The TLV WR1:1 provides increased cardiac protection for older workers, as compared with the CON and the WR3:1.