Inhomogeneities of population and saturation are responsible for lens effects in gas lasers. They give rise to an asymmetric Lamb dip. Generally, one of these inhomogeneities is predominant and thus determines the sense of the asymmetry. However, since the two kinds of inhomogeneities are modified by the geometric parameters, it may happen that they give lens effects of the same magnitude, but of opposite sign. In such a case of competing effects which cancel each other, the line shape may vary strongly when a parameter is changed. A laser of this kind will be called «sensitive». In this paper we use as an example a quasi-hemispheric He-Ne laser whose length (57 cm) is almost equal to the radius of curvature (60 cm) of the concave mirror. We show experimentally that the sense of the asymmetry may be changed by the discharge current and that the line shape also depends on the diameter of the diaphragm. This behaviour reveals the competition between the two effects. We numerically calculate the line shapes by using methods which we have developed recently. Qualitatively, we find these results in excellent agreement with the experiments Laser «sensible»: un laser a gaz ou les inhomogeneites de saturation et de population entrent en competition en fonction des parametres geometriques du laser: la forme de raie peut varier beaucoup pour un faible changement de parametres. Exemple d'un laser quasi hemispherique a Me-Ne dont la longueur est presque egale au rayon de courbure du miroir concave