Serum samples were collected from 24 platyrrhines of seven diurnal species housed with outdoor access at Bristol Zoo Gardens (United Kingdom) to test 25-hydroxyvitamin D₃ (25OHD₃) levels as part of the veterinary department's preventative health care program. Samples were collected in August 2008 (summer) and January 2009 (winter) to examine the effect of season on 25OHD₃ levels. Dietary levels of vitamin D₃ remained the same throughout the study period and fell within the range of 2000–4000 IU/kg dry matter, in accordance with current primate guidelines. Statistical analysis showed that there was no significant difference between the platyrrhines' summer 25OHD₃ values (range, <4.0–>150.0 μg/L) and winter 25OHD₃ values (range, <4.0–80.1 μg/L). However, ultraviolet B (UVB) measurements taken at the zoo during the study period confirmed that UVB levels were significantly higher in summer (mean reading for 1200–1300 hours GMT time period, 153.8 μW/cm²) compared with winter (mean reading for 1200–1300 hours GMT time period, 19.4 μW/cm²). The 25OHD₃ levels measured were generally found to be low compared with previously published values from healthy captive and wild platyrrhines.