There has been some suggestion in the literature that base-in vergence ranges should be recorded before base-out ranges. The rationale for this proposal is that the convergence responses stimulated during the base-out measurements may produce adaptation of tonic vergence (TV), thereby biasing the subsequent base-in values. However, this hypothesis runs counter to some models of vergence adaptation, which indicate that changes in TV will only result following a sustained vergence response of relatively constant magnitude. Accordingly, the aim of the present study was to compare distance (6m) base-in vergence ranges measured before and after the base-out ranges in 10 visually-normal subjects. Additionally, TV was measured prior to, and following each vergence range measurement, be assessing the distance phoria recorded through 0.5mm pinholes. The results indicated that the determination of the base-out ranges produced a significant increase in TV. This finding demonstrates that adaptation can be produced by a vergence stimulus whose magnitude varies considerably over time. In addition, when base-in measurements were recorded after base-out testing, the value of its recovery component was significantly reduced. However, there was no significant difference between the base-in break values obtained either before or after the base-out measurements. Thus this study provides evidence that vergence range testing can produce variations in TV, and that base-in vergence ranges should be testing before the base-out measurements.