We analyse the horizontal and vertical spread of a continuous, point-source plume in two model urban canopies. Concentration measurements were obtained in a water tunnel utilizing fast response micro-conductivity probes. Experiments were undertaken for model urban canopies of uniform arrays with buildings of height, H, width wb, length B and building aspect ratios H/wb=1 and 3 for which the values of the building areal density parameters (lambda)p and(lambda)f = 0.19 for the H/wb = 1 urban canopy, (lambda)p = 0.19 and (lambda)f = 0.56 for the H/wb = 3 urban canopy. The direction of the mean flow is parallel to the canyons. Non-dimensional lateral scales of the plume are ~50% greater for the shorter canopy. Examination of the lateral profiles suggested delineation of the ground level concentration field into near and far fields for both canopies. The longitudinal extent of the near field, defined as the region of linear lateral growth of the plume, is given by xL ~ 7.5B. The ratio of the horizontal to vertical scales were different in the horizontal and vertical directions for the two model canopies with width ~ height for the H/wb = 1 canopy, but width ~ (0.5)height for the H/wb = 3 canopy. Thus plume geometry is isotropic for the short canopy and anisotropic for the tall canopy. Values of non-dimensional dispersion coefficient Kc(wbUb)-1 increase from the ground level values of O (0.1) to peak values of O (1) just above the building height.