We develop a model of the combined thermal and mechanical evolution of diapirs in the mantle of Venus. The diapir is treated as an oblate spheroid rising through a viscous fluid and ultimately impinging on a rigid overlaying lid. Drag forces imposed by the lid cause the diapir to spread and flatten as it rises. We parameterize the heat loss from the rising diapir using a Nusselt number formulation and treat the resulting loss in buoyancy in our flow calculation. The model predicts the evolution timescale and degree of flattening of a diapir as it rises, as well as the stresses exerted on the underside of the lithosphere. In order to explore diapir behavior further, we check our analytical model against the predictions of the finite element code MANTLE. From the combined results of these models and observations of Venusian coronae, we are able to make a number of inferences about Venusian diapirism and mantle properties. We find that the diapirs responsible for formation of Venusian coronae have an initial size distribution extending from about 30 to 100 km in radius. Typical evolution timescales for these diapirs are 30--50 Myr. If Venus' global average resurfacing age is 300--500 Myr, then our results are consistent with a rate of diapirism that has been roughly constant over this period, and with an effective mantle viscosity of 1021 Pa s.¿ 1997 American Geophysical Union