We present acoustic and articulatory data from an experiment designed to test the phonetic variability of rhotics in Brazilian Portuguese, focusing on the São Paulo variety. Ultrasound tongue imaging was used to examine the realisation of rhotics in a range of phonological environments. Our analysis reveals that word-initial and intervocalic fricatives are acoustically and articulatorily distinct for most speakers. We attribute a tendency for utterance-initial fricatives to display longer duration, less voicing, and greater tongue-dorsum displacement than word-medial intervocalic counterparts to phonetic enhancement at the site of a major prosodic boundary. Similarly, rhotic taps in utterance-final position show a tendency for devoicing and frication (aspiration or assibilation) speaker-dependently. By comparison, word-medial pre-consonantal and intervocalic taps are characterised by shorter durations and greater voicing: hence, a pattern of phonetic reduction in prosodically weaker environments. We relate our findings to theoretical debates around the phonological status of rhotics in Portuguese. Whilst not providing conclusive proof in favour of any one particular approach, our results highlight the need to recognise the reality of prosodically driven strengthening in developing a full account of rhotic variation in the variety.