We propose a procedure for the determination of lactic acid or lactate in biological fluids, which is simple and low cost with respect to the known methodologies. This procedure can be used by unskilled personnel and it can be implemented in disposable test strips and portable measure devices. The procedure is based on the photochemical reaction of lactic acid with Fe(III), which is reduced to Fe(II) when irradiated with UV light. The Fe(II), produced proportionally to the amount of lactate in the sample, is determined electrochemically or using its reaction with a colored complexing agent, such as 1,10-phenanthroline, and determined by spectrophotometry. This method is advantageous both for the low cost of the reagents employed, and for their short and long term stability.