Several endurance sports, e.g. road cycling, have a varying intensity profile during competition. At present, few laboratory tests take this intensity profile into consideration. Thus, the purpose of this study was to examine the prognostic value of heart rate (HR), lactate (La⁻¹), potassium (K⁺), and respiratory exchange ratio (RER) performance at an exhausting cycling exercise with varying intensity. Eight national level cyclists performed two cycle tests each on a cycle ergometer: (1) a incremental test to establish VO_2max, maximum power (W _max), and lactate threshold (VO_2LT), and (2) a variable intensity protocol (VIP). Exercise intensity for the VIP was based upon the VO_2max obtained during the incremental test. The VIP consisted of six high intense (HI) workloads at 90% of VO_2max for 3 min each, interspersed by five middle intense (MI) workloads at 70% of VO_2max for 6 min each. VO₂ and HR were continuously measured throughout the tests. Venous blood samples were taken before, during, and after the test. Increases in HR, La^-, K⁺, and RER were observed when workload changed from MI to HI workload (P < 0.05). Potassium and RER decreased after transition from HI to MI workloads (P < 0.05). There was a negative correlation between time to exhaustion and decrease in La^- concentration during the first MI (r = −0.714; P = 0.047). Furthermore, time to exhaustion correlated with VO_2LT calculated from the ramp test (r = 0.738; P = 0.037). Our results suggest that the magnitude of decrease of La⁻¹ between the first HI workload and the consecutive MI workload could predict performance during prolonged exercise with variable intensity.