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A number of works has been devoted to the measurement of excited species concentration in flames by emission spectroscopy. Since the pioneering work of KASKAN of OH, only a few laboratories are involved in measurement by absorption spectroscopy. This paper presents the measurements of OH, CN, NH number density profiles in a low pressure (25 torrs) methane-oxygen flat flame, doped with 6 % $NH3$. The first part deals with experiments performed with a conventional device: continuous light source and analysis of the absorption spectra with a mospatial resolution only a single path in the 7 cm dia flame is used. It is shown that an actual spatial resolution needed. Total absorption Mitchell-Zemansky’s procedure is used to obtain the populations of some rotational levels ; then rotational temperatures can be deduced. Good agreement is found between the rotational temperatures profile and the translational one. The second part of the paper presents a comparison of OH number density profiles measured in the flame with the previous device and with absorption measurement, and induced fluorescence measurement performed with tunable pulsed laser. In these experiments the spectreal analysis is made by the laser itself, spatial resolution and detect ability limits are improved.