This article aims to widen the empirical evidence about the determinants of Spanish academics' publication productivity across fields of study. We use the Spanish Survey on Human Resources in Science and Technology addressed to Spanish resident PhDs employed in Spanish universities as academics. Productivity is measured as the total number of publications in a three-year period. We show how personal and academic variables explain differences in productivity within universities and fields of studies and across fields of research. Female workers report lower productivity than their male counterparts, but family responsibilities do not explain this gender gap. The type of contract and tenure or rank do not seem to have any influence on productivity. Researchers seeking professional promotion rather than altruism or personal satisfaction are more productive and young scholars publish more than their older counterparts. Additionally, we find a certain research-teaching trade-off and some nuances in the predictors of publication productivity across birth cohorts and fields of study. Finally, international cooperation is one of the most relevant determinants of the number of publications, regardless of the birth cohort. The institutional context in the Spanish research system as regards requirements for promotion and the assessment of research outcomes may contribute to the understanding and interpretation of our results.