Pulsatile and circadian patterns of PRL release were studied in 11 insulin-dependent diabetic men by sampling blood every 10 min for 24 h and comparing the results to those obtained in 12 normal nondiabetic men. The diabetic men had a mean (+/- SE) 24-h serum PRL concentration of 5.5 +/- 0.42 micrograms/L, which was significantly lower than that in the nondiabetic men (9.3 +/- 0.86; P = 0.0008). Quantitative Cluster analysis of pulsatile PRL time series revealed a normal pulse frequency, but decreased maximal peak amplitude (6.6 +/- 0.5 vs. 11.8 +/- 1.1 micrograms/L; P = 0.0009), peak increment (2.6 +/- 0.24 vs. 4.0 +/- 0.3 micrograms/L; P = 0.009), peak area (126 +/- 15 vs. 192 +/- 19 micrograms/L.min; P = 0.03), and interpulse valley mean concentration (4.8 +/- 0.4 vs. 8.6 +/- 1.2 micrograms/L; P = 0.0007). PRL pulse incremental amplitude correlated significantly (r2 = 0.577; P = 0.007) and negatively with duration of disease. Fourier analysis disclosed a normal circadian rhythm of PRL release in diabetic men, with a mean circadian amplitude of 1.5 micrograms/L +/- 0.31, which peaked at 0201 h +/- 89 min (+/- SE). In summary, we have demonstrated significantly reduced mean 24-h serum PRL concentrations in men with poorly controlled insulin-dependent diabetes mellitus. The concomitant suppression of spontaneous PRL pulse amplitude, peak increment, and interpulse valley mean concentrations in the presence of normal pulse frequency is consistent with a reduced mass of PRL secreted per burst and/or accelerated metabolic clearance of PRL in men with type I diabetes mellitus.