We used time-of-flight photocurrent measurements to determine the role of grain boundaries in charge carrier transport in thin layers of methyl ammonium lead iodide (CH3NH3PbI3). The measurement results were compared to Kinetic Monte Carlo simulations, based on a transport model, which disentangles the transport within crystallites and hopping across grain boundaries. The observed mobilities of electrons are in the order similar to 2.5 x 10(-1) cm(2) V- 1 s(-1) . The hopping across grains is modeled with an Arrhenius-type probability rate, characterized by activation energy (E-a). It was found that the E-a estimated from the slope of a mobility-temperature dependence is in the range of similar to 56-70 meV. The factors contributing to E-a are shunting pathways and the grain-size variations including energy level misalignments at the grain boundaries. These results represent a step toward a design of novel windowless organic-inorganic perovskite solar cells.