The relationship between muscle and blood lactate levels during progressively step-wise incrementing cycle exercise has been investigated in 10 male subjects. Steps between power outputs during exercise were 50 W and each stage, from loadless pedalling until voluntary exhaustion, lasted 4 min. Blood samples and biopsies (m. vastus lateralis) were taken for lactate determination at each power output beginning with the exercise intensity perceived by the subject as being "rather moderate". The ratio muscle:blood lactate was greater than one at all power outputs and increased most markedly at the power output closest to that eliciting 4 mmol x 1(-1) blood lactate (WOBLA). At WOBLA, blood lactate was positively correlated to muscle lactate concentrations which covaried widely among subjects (mean 8.3, range 4.5-14.4 mmol x kg-1 wet weight). Muscles fibres from the WOBLA biopsy in 6 subjects were dissected out and identified as fast twitch (FT) or slow twitch (ST). No significant difference in lactate concentration was observed between pools of FT or ST fibres.