Passive elevation of muscle temperature (Tm) prior to exercise has previously been estimated to increase anaerobic energy turnover during heavy cycling exercise at 60 revs.min-1 (Ferguson et al., 2002). The aim of this study was, through measurement of homogenate and single fibre muscle metabolites, to clarify the effect of passively increasing Tm on anaerobic energy turnover and to determine whether any differences were due to a fibre type specific effect of increasing muscle temperature. Following local ethics committee approval 6 male subjects (age 25 ± 3 yrs, height 1.85 ± 0.06 m, body mass 87 ± 16 kg; means ± SD) volunteered to performed a 6 min bout of heavy cycling exercise (half way between lactate threshold and VO2max) under conditions of normal (N: Tm= 34.7 ± 1.1 °C) and elevated (ET: Tm= 37.4 ± 0.2 °C) Tm at a pedal rate of 60 revs.min-1. Muscle biopsies were obtained at 0, 2 and 6 min. Anaerobic energy turnover was calculated through measure of muscle ATP, PCr and lactate. Approximately 300 single muscle fibres were dissected, from 2 subjects, and analysed for MHC composition (SDS-PAGE) and PCr content (Wibom et al, 1991). Fibres were classified, according to the dominant MHC, as: type I, IIA, and IIAX. Statistical analyses were performed using two-way repeated measures ANOVA and Bonferroni corrected paired t-tests where appropriate. Muscle metabolite content was the same between conditions at rest and following 6 min of exercise, however at 2 min there was a lower PCr content (35.0 ± 8.8 (ET) vs 40.9 ± 8.3 (N) mmol.kg.-1 (dm): P<0.05) and elevated lactate content (19.4 ± 3.7 (ET) vs 14.0 ± 3.8 (N) mmol.kg.-1 (dm): P<0.05) in ET compared to N. Anaerobic energy turnover was 298.6 ± 79.2 J.s-1 in N and 384.4 ± 108.9 J.s-1 in E, over the first two minutes of exercise, a 29 % increase (P<0.05) during ET. This increase in anaerobic energy turnover was associated with a tendency (P=0.06) for a lower PCr content in type I fibres after 2 min of exercise in ET (27.0 ± 11.4 mmol.kg.-1 (dm)) compared to N (21.3 ± 9.9 mmol.kg.-1 (dm)). There were no differences (P>0.05) in PCr content at any other time points or in IIA and IIX fibres. We have demonstrated that passive elevation of Tm leads to an increase in anaerobic energy turnover in the initial period of heavy exercise possibly due to a greater PCr utilisation in type I fibres, although further work is required to confirm this.