Xenodiagnoses were performed every 3 hours using 10 Triatoma infestans 3rd instar for 24 to 72 hours, in 18 chronic chagasics with positive serology and/or xenodiagnosis. There was no statistically significant difference in the positivity of assays performed during the day (9:00 to 18:00 h) compared to those performed at night (21:00 to 6:00 h), (chi 2 = 0.1526 p = 0.696). Xenodiagnosis was performed in ten of the patients for 13 successive days but there was no periodicity detected in the positive assays.