We compared the influence of open-book extended duration versus closed book time-limited format on reliability and validity of written assessments of pharmacology learning outcomes within our medical and dental courses. Our dental cohort undertake a mid-year test (30xfree-response short answer to a question, SAQ) and end-of-year paper (4xSAQ, 1xessay, 1xcase) in pharmacology. For our first year medical cohort, pharmacology is integrated within a larger course, contributing 20xclinical vignette questions (to select the single best answer (SBA) to each question from a choice of 5 plausible answers) to a mid-year test and 3-5xSAQ to an end-of-year paper. Our experience indicates that SAQ are as reliable as SBA for closed-book time-limited assessments; reliability correlates with number of questions employed. We have found good correlation between mid-year and end-of-year performance (predictive validity), between questions (factorial validity) and between pharmacology and other subjects within the assessment (concurrent validity). Adoption of open-book extended duration assessments resulted in only modest reduction in reliability and validity.