Well-designed peer marking procedures can give students rapid and accurate feedback on their work. Harris (2011) described and evaluated such a procedure for hand-marking ‘pro-forma’ physiology laboratory reports in a large (ca. 200) first year undergraduate class. Staff and student feedback on the approach was positive but its format made it difficult for the reports to include free text and many students disliked the lack of anonymity in peer marking. We have now adapted the procedure so that anonymous peer marking of reports that include free text can be carried out online within eBiolabs, a web-based system developed to support and assess laboratory-based work (Hughes et al, 2012). Students upload physiological data, including numerical analysis and graphs, obtained in the lab plus free text answers to related questions. All work must be submitted within one week of the practical class. Each student’s work is assigned randomly to two peers who allocate marks with reference to marking criteria and specimen answers that are released online, and explained verbally, immediately after the submission deadline but before the peer marking opens. Peer marking (anonymous to both the writer and marker) must then be completed within one week. Staff are able to review all uploaded work, the identity of student markers and the marks awarded. Anomalous marking is checked and is moderated if the two peer marks differ by more than 15%. Finalised marks are made available to students within one week of the marking deadline and contribute 2% to the final unit mark. The system has now been used for 5 years to peer-assess a first year renal physiology practical report submitted by ca. 200 BSc students. In the first 3 years a range of technical and student compliance issues were encountered but these have now been largely resolved. In 2013-14 and 2014-15, 68% and 74% respectively of the submitted reports required no staff moderation and students were awarded the average of the two peer marks. Furthermore, 86% (13-14) and 91% (14-15) of the student cohort were totally compliant with the process. Examples of student non-compliance included failure to upload their own report and/or to peer-mark the reports that were allocated to them. Feedback from ca. 75 students in 13-14 showed that 80% agreed that they received a fair mark for their work whilst 64% agreed that evaluating other students’ work was helpful for their own learning. We conclude that online peer assessment of laboratory reports can give students rapid, anonymous and fair feedback on work that includes free text, and that around two-thirds of students find the process helpful for their learning. Although generally efficient in staff time, some intervention is required to resolve issues of peer marker non-compliance or inaccuracies in student marking.