To further our understanding of the nature of control mechanisms involved in the adaptation of exercising muscle hyperemia, we examined forearm hemodynamics in response to step increases in work rate. 7 subjects performed rhythmic dynamic forearm exercise (1 s/2 s contraction/relaxation duty cycle) under two exercise conditions: small step 1 (step increase from rest to 40% peak forearm vascular conductance (FVC, ml/min/100 mmHg)) for 5 min followed by small step 2 (further increase to 80% peak FVC for 5 min); large step (step increase from rest to 80% peak FVC for 5 min). FVC ([Doppler ultrasound forearm blood flow ÷ arterial tonometry blood pressure]x 100) data were fit with a 2 (small step 1) and 3 (small step 2, large step) component exponential to quantify the rapid phase I, slower phase II and very slow phase III components of the response as appropriate. Neither the time delay (TD (s)) nor the time constant ( (s)) of the FVC response of either the rapid phase I or slower phase II response was affected by the magnitude of the exercise step when exercise was initiated from rest. Initiating an increase in exercise intensity from an exercise baseline resulted in a longer TD and slightly slower for the phase I response. It also resulted in a longer TD for the phase II response but did not slow its . For the rapid phase I increase in FVC: Rest-80% gain was greater than either Rest-40% or 40-80% transitions (175.6 ±22.7 vs. 96.6 ±17.1 vs. 97.8 ±14.2 ml/min/100 mmHg , P < 0.001), but represented the same proportion of the Phase I + Phase II gain across all transitions (Rest-40% vs. Rest - 80% vs. 40-80%; 57% vs. 56% vs. 57%, P=0.975). These data suggest 1) dynamic linearity of the phase I and II control systems when small muscle mass exercise is initiated from rest, with the phase II response retaining dynamic linearity in a transition from a baseline of exercise, and 2) the combination of mechanisms contributing to phase I result in the same relative response magnitude regardless of the size of the step increase in exercise intensity or the baseline from which it is initiated.