[en] The studies on the effect of magnetic field and external pressure on temperature dependant electrical resistivity behaviour of polycrystalline MnAs have been reported. At ambient pressure, ρ(T) shows a first order magnetic transition associated with change in sign of the temperature coefficient of resistivity from positive in the ferromagnetic (FM) phase to negative in the paramagnetic (PM) phase. The magneto resistance is negative and shows a peak at the FM transition temperature (T _C). The first order hysteresis width decreases with increase in magnetic field and the intersection of extrapolated linear variations of T _C with field for the cooling and warming cycles enabled determination of the tricritical point. At high pressures, ρ(T) displays non monotonic variation exhibiting a low temperature minimum $(T_{min}^L)$ and a high temperature maximum $(T_{max}^H)$ accompanying broad thermal hysteresis above $T_{min}^L.$ It is surmised that spin disorder scattering is responsible for the resistivity behaviour above $T_{min}^L$ and the essential features of ρ(T) are qualitatively explained using Kasuya theoretical model. Below the $T_{min}^L,$ ρ(T) follows linear logarithmic temperature dependence similar to the effect occurring due to Kondo type of scattering of conduction electrons with localised moments. (paper)