The primary purpose of this study was to determine the maximal cardiorespiratory responses of 48, 5- to 6-year-old children (24 girls and 24 boys), who were tested on a treadmill (TM) and an electronically braked cycle ergometer (CE). We also examined the percentage of boys and girls who were able to achieve the criteria for reaching maximal oxygen consumption (VO2max) on each testing mode. After an orientation period, each child was tested twice (once on the TM and once on the CE), with an interval of 1 week between tests. VO2max was measured during progressive, all out, continuous TM and CE tests using standardized procedures. A 2 x 2 analysis of variance test (genderxexercise mode) with Bonferroni adjustment revealed the following: (1) there were no gender differences in any of the measured dependent variables, (2) the children produced a significantly higher relative VO2max (ml x kg(-1) x min(-1) and a higher absolute VO2max (l x min(-1)) on the TM than on the CE (P < 0.001), (3) the children produced a higher heart rate on the TM than on the CE (P < 0.001), (4) the CE generated significantly higher values for respiratory exchange ratio (P < 0.001), (5) the criteria necessary to establish a maximal exercise effort were achieved on both the TM and the CE, and (6) all of the children reached two of the criteria associated with a maximal effort, while only 46% of the children reached three criterion measures. There were no significant differences in the attainment of criterion measures between the TM and the CE. The results of this study indicate a lack of gender differences in maximal exercise testing in 5- to 6-year-old children, and that both the CE and the TM are effective modes of maximal cardiorespiratory testing in this age group.