In dynamic exercise of increasing intensity baroreflex sensitivity (BRS), measured by the sequence technique (Iellamo, 1998), or gain, by neck pressure change (Fadel et al. 2003), has been found to decrease progressively. However, baroreflex control in isometric exercise is less well understood (Carrington & White, 2001). With progressive increases in isometric force, the interaction of greater central command, mechanoreceptor stimulation and metaboreflex activation make it difficult to predict BRS. This study examined BRS during isometric exercise of increasing intensity. Ten subjects (7 male, mean±S.E.M.) aged 22.4±0.9years sat in an isokinetic dynamometer with the right knee flexed at 150 deg and the foot attached with the lower leg horizontal. Heart rate (HR) and blood pressure (BP) were measured using an ECG and Finapres. Respiratory phase was measured using a strain gauge around the chest and respiratory rate was maintained at oeupnoeic frequency by breathing to a metronome. After 290s of rest, a cuff was inflated around the right thigh to 200mmHg. Subjects then either rested (0% trial) or performed 90s of isometric plantarflexor exercise at 30, 50 or 70% of maximum voluntary contractile force (MVC). Mean resting HRs were 70±3, 68±2, 73±4 and 73±3 b min^-1 prior to the 0, 30, 50 and 70% trials, respectively, and were 70±4, 85±4, 100±5 and 119±7 b min^-1 by end of rest or exercise, respectively. Mean resting BPs were 86±3, 89±3, 93±3 and 95±3 mmHg prior to the 0, 30, 50 and 70% trials, respectively, and were 88±3, 109±3, 126±4 and 141±4mmHg by end of rest or exercise, respectively. Regression equations produced by sequence analysis of R-R intervals (Y axis) and systolic blood pressure (SBP) (X axis) gave mean slope and intercept values shown in Table 1. There were no significant differences between trials for slopes and intercepts at rest (repeated measures ANOVA). However, exercise decreased slope and increased intercept from rest (P<0.05) and these changes became progressively greater with increasing exercise intensity (P<0.05, post hoc LSD test). This study has shown that BRS decreases progressively with increasing intensity of isometric exercise, with a concomitant rightward shift of the R-R v SBP relationship. Whether this progressive change is due to increased central command, increased muscle mechanoreceptor stimulation or metaboreflex activation requires further study.