A study was undertaken to estimate the amount oxygen that can diffuse across the placenta of the passive smoking mothers in an Indo-Asian population. A total of 24 human term placentas, 11 from mothers exposed to tobacco smoke and 13 from those not exposed to tobacco smoke were collected for estimating weight, surface area and volume term placentas. Five placentas from each of the groups were used for estimating the various parameters for calculating the oxygen-diffusing conductance. A random sampling technique at 7 levels of tissue preparation was adopted.Cotyledons were cut into blocks and each block was further cut into upper (maternal), middle (foeto-maternal) and lower (foetal) components. Semithin sections, 1 m thick, stained using a modified toluidine was viewed and the images were analysed. A Merz grid was used to estimate the vascular volumes, exchange surface areas and tissue diffusion distances for calculation the oxygen-diffusing conductance of the placenta. These were combined with physicochemical quantities (oxygen-haemoglobin reaction rates and tissues oxygen diffusion coefficients) obtained from literature in order to estimate the partial conductance of the six tissue compartments of the oxygen pathway. The present study demonstrates that there is no significant difference in the total oxygen-diffusing conductance in the placenta of passive smokers and those not exposed to tobacco smoke. However, the significant increase in the foetal capillaries and foetal erythrocytes conductance suggests that an adaptive mechanism in the foetal part of the placenta may be occurring to ensure adequate oxygen to the foetus