In a recent study (Widegren U, Hickner RC, Lindström A, Ungerstedt U, Hjemdahl P, Jorfeldt L & Henriksson J, unpublished observations), we found that i.v. infusion of adrenaline resulted in a significant decrease in the blood flow of human gastrocnemius muscle when blood flow was measured by the microdialysis ethanol technique (Hickner et al. 1992). This contrasted with the significant increase in blood flow, which was detected when blood flow was instead measured by the ¹³³Xe clearance technique or by venous occlusion pletysmography. We hypothesized that the observed discrepancy in the results may be secondary to the more extensive local trauma involved in the microdialysis ethanol technique compared with the ¹³³Xe clearance and the pletysmography methods. In the present study, this hypothesis was tested by using ¹³³Xe clearance to measure blood flow close to an already inserted microdialysis catheter, both in the basal state and during i.v. infusion of adrenaline (0.1 nmol kg^-1 min^-1) or vehicle only. This was done by administering the ¹³³Xe through a fine tube that had been inserted along with the microdialysis catheter. Values (n=8), given as means ± S.E.M., were statistically tested using two-way repeated measures ANOVA with two within factors, treatment (adrenaline – placebo infusion) and effect (basal – infusion values). Although adrenaline caused a significant increase in heart rate, and a widening of the pulse pressure, the blood flow close to the microdialysis catheter was decreased (to an average value of 72.8 ± 5.4% of basal; p<0.01). This decrease was significantly larger (p<0.05) than the change that occurred during i.v. infusion of vehicle only (to 87.7 ± 2.3% of basal, p<0.01). The decrease in the control experiment was possibly due to the cold temperature of the infusion solutions. In conclusion, the vasodilatation in skeletal muscle normally observed in response to intravenously infused adrenaline is converted into vasoconstriction by the influence of an inserted microdialysis catheter. We propose that a small local trauma may change the balance between vasoconstrictive and vasodilatatory effects of adrenergic activation in skeletal muscle.