This paper integrates the analysis of the decision to undertake R&D activities with the analysis of the decision on the level of the R&D investment when this investment is carried out. The theoretical framework assumes the existence of minimum required R&D expenditures, brought about the indivisibility of some resources. This assumption, combined with demand characteristics and technological opportunities, determine threshold levels of R&D expenditures under which firms do not find profitable to invest. This framework leads naturally to a Tobit-type econometric model aimed at estimating thresholds, which we apply to a Spanish manufacturing representative micropanel sample that includes observations on more that 2,000 firms of all size, many without R&D expenditures. Our conclusions are that sizable thresholds exist, which are sistematically related to a list of demand and technological factors. The results strongly suggest that there is an advantage of the biggest firms in undertaking innovative activites.