Patients with fibromyalgia often complain of fatigue and pain during exercise and of worsening of pain days after exercise. The aim of the study described here was to determine if abnormal changes in potassium or lactate could be observed during an exercise test in fibromyalgia. Whether an abnormal incline in plasma creatine kinase or myoglobin could be observed days after the test was studied also. Fifteen female fibromyalgia patients and 15 age- and sex-matched controls performed a stepwise incremental maximal bicycle-ergometer test. Blood samples were collected from a catheter in a cubital vein. The changes in heart rate, potassium levels, and haematocrit during the exercise test were similar in the two groups. The maximal obtained lactate concentration was 4.2 mmol l-1 (3.5-5.6) in the patients as compared to 4.9 mmol l-1 (3.9-5.9) in the controls (NS). The estimated anaerobic threshold of 2 mmol l-1 was reached at a heart rate of 124 min-1 in the patients with fibromyalgia as compared to 140 min-1 in the controls (P = 0.02). In relation to workload, the patients scored higher on a Borg scale for perceived exertion during exercise, but if the Borg score was related to lactate no significant difference was found. The patients reported 86% and 79% of maximal pain in the thighs on the visual analogue scale 1 and 2 days after the test, but the creatine kinase and myoglobin concentrations were not increased.